Add permission callback for user-initated downloads (fixes issue #3183)

This change adds a CefDownloadHandler::CanDownload callback for optionally
blocking user-initiated downloads (e.g. alt + link click or link click that
returns a `Content-Disposition: attachment` response from the server).

To test:
- Run `ceftests --gtest_filter=DownloadTest.*`.
- Run `cefclient --hide-controls`. User-initiated downloads will be blocked.

libcef/browser/alloy/alloy_browser_host_impl.cc

@@ -1225,6 +1225,13 @@ bool AlloyBrowserHostImpl::HandleContextMenu(
   return HandleContextMenu(web_contents(), params);
 }
 
+void AlloyBrowserHostImpl::CanDownload(
+    const GURL& url,
+    const std::string& request_method,
+    base::OnceCallback<void(bool)> callback) {
+  contents_delegate_->CanDownload(url, request_method, std::move(callback));
+}
+
 KeyboardEventProcessingResult AlloyBrowserHostImpl::PreHandleKeyboardEvent(
     content::WebContents* source,
     const content::NativeWebKeyboardEvent& event) {

libcef/browser/alloy/alloy_browser_host_impl.h

@@ -225,6 +225,9 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
   bool TakeFocus(content::WebContents* source, bool reverse) override;
   bool HandleContextMenu(content::RenderFrameHost& render_frame_host,
                          const content::ContextMenuParams& params) override;
+  void CanDownload(const GURL& url,
+                   const std::string& request_method,
+                   base::OnceCallback<void(bool)> callback) override;
   content::KeyboardEventProcessingResult PreHandleKeyboardEvent(
       content::WebContents* source,
       const content::NativeWebKeyboardEvent& event) override;

libcef/browser/browser_contents_delegate.cc

@@ -180,6 +180,23 @@ void CefBrowserContentsDelegate::ExitFullscreenModeForTab(
   OnFullscreenModeChange(/*fullscreen=*/false);
 }
 
+void CefBrowserContentsDelegate::CanDownload(
+    const GURL& url,
+    const std::string& request_method,
+    base::OnceCallback<void(bool)> callback) {
+  bool allow = true;
+
+  if (auto delegate = platform_delegate()) {
+    if (auto c = client()) {
+      if (auto handler = c->GetDownloadHandler()) {
+        allow = handler->CanDownload(browser(), url.spec(), request_method);
+      }
+    }
+  }
+
+  std::move(callback).Run(allow);
+}
+
 KeyboardEventProcessingResult
 CefBrowserContentsDelegate::PreHandleKeyboardEvent(
     content::WebContents* source,

libcef/browser/browser_contents_delegate.h

@@ -101,6 +101,9 @@ class CefBrowserContentsDelegate : public content::WebContentsDelegate,
       content::RenderFrameHost* requesting_frame,
       const blink::mojom::FullscreenOptions& options) override;
   void ExitFullscreenModeForTab(content::WebContents* web_contents) override;
+  void CanDownload(const GURL& url,
+                   const std::string& request_method,
+                   base::OnceCallback<void(bool)> callback) override;
   content::KeyboardEventProcessingResult PreHandleKeyboardEvent(
       content::WebContents* source,
       const content::NativeWebKeyboardEvent& event) override;

libcef/browser/chrome/chrome_browser_delegate.cc

@@ -235,6 +235,20 @@ void ChromeBrowserDelegate::ExitFullscreenModeForTab(
   }
 }
 
+void ChromeBrowserDelegate::CanDownload(
+    const GURL& url,
+    const std::string& request_method,
+    base::OnceCallback<void(bool)> callback) {
+  auto source = browser_->tab_strip_model()->GetActiveWebContents();
+  DCHECK(source);
+
+  if (auto delegate = GetDelegateForWebContents(source)) {
+    delegate->CanDownload(url, request_method, std::move(callback));
+    return;
+  }
+  std::move(callback).Run(true);
+}
+
 KeyboardEventProcessingResult ChromeBrowserDelegate::PreHandleKeyboardEvent(
     content::WebContents* source,
     const content::NativeWebKeyboardEvent& event) {

libcef/browser/chrome/chrome_browser_delegate.h

@@ -86,6 +86,9 @@ class ChromeBrowserDelegate : public cef::BrowserDelegate {
       content::RenderFrameHost* requesting_frame,
       const blink::mojom::FullscreenOptions& options) override;
   void ExitFullscreenModeForTab(content::WebContents* web_contents) override;
+  void CanDownload(const GURL& url,
+                   const std::string& request_method,
+                   base::OnceCallback<void(bool)> callback) override;
   content::KeyboardEventProcessingResult PreHandleKeyboardEvent(
       content::WebContents* source,
       const content::NativeWebKeyboardEvent& event) override;