Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

libcef/browser/extensions/browser_extensions_util.h

@@ -9,13 +9,18 @@
 
 #include "libcef/browser/browser_host_impl.h"
 
+#include "url/gurl.h"
+
 namespace content {
+class BrowserContext;
 class RenderViewHost;
 class WebContents;
-}
+}  // namespace content
 
 namespace extensions {
 
+class Extension;
+
 // Returns the full-page guest WebContents for the specified |owner|, if any.
 content::WebContents* GetFullPageGuestForOwnerContents(
     content::WebContents* owner);
@@ -47,6 +52,18 @@ CefRefPtr<CefBrowserHostImpl> GetOwnerBrowserForHost(
     content::RenderFrameHost* host,
     bool* is_guest_view);
 
+// Returns the browser matching |tab_id| and |browser_context|. Returns false if
+// |tab_id| is -1 or a matching browser cannot be found within
+// |browser_context|. Similar in concept to ExtensionTabUtil::GetTabById.
+CefRefPtr<CefBrowserHostImpl> GetBrowserForTabId(
+    int tab_id,
+    content::BrowserContext* browser_context);
+
+// Returns the extension associated with |url| in |profile|. Returns nullptr
+// if the extension does not exist.
+const Extension* GetExtensionForUrl(content::BrowserContext* browser_context,
+                                    const GURL& url);
+
 }  // namespace extensions
 
 #endif  // CEF_LIBCEF_BROWSER_EXTENSIONS_BROWSER_EXTENSIONS_UTIL_H_