chrome: Support Chrome and DevTools command execution (fixes #3282, fixes #3600)

Add new CefBrowserHost::[Can]ExecuteChromeCommand methods for executing
arbitrary Chrome commands.

Add support for existing CefBrowserHost::ShowDevTools, CloseDevTools and
HasDevTools methods.

DevTools windows now support the same Views callbacks as normal popup
windows with the new CefLifeSpanHandler::OnBeforeDevToolsPopup callback
as the DevTools-specific equivalent of OnBeforePopup.

Always create DevTools as an undocked window to support use of
ShowDevTools with default Chrome browser windows.

To test:

Run `ceftests --enable-chrome-runtime [--use-views]
     --gtest_filter=V8Test.OnUncaughtExceptionDevTools`

OR:

1. Run `cefclient --enable-chrome-runtime [--use-native]`
2. Select "Show DevTools", "Close DevTools" or "Inspect" from the
   right-click menu.
3. Notice that the DevTools window is Views-hosted (or native-hosted)
   and works as expected.

Add --use-default-popup to get a default styled popup in step 3.

libcef/browser/browser_host_base.h

@@ -94,6 +94,23 @@ struct CefBrowserCreateParams {
       extensions::mojom::ViewType::kInvalid;
 };
 
+// Parameters passed to ShowDevToolsOnUIThread().
+struct CefShowDevToolsParams {
+  CefShowDevToolsParams(const CefWindowInfo& windowInfo,
+                        CefRefPtr<CefClient> client,
+                        const CefBrowserSettings& settings,
+                        const CefPoint& inspect_element_at)
+      : window_info_(windowInfo),
+        client_(client),
+        settings_(settings),
+        inspect_element_at_(inspect_element_at) {}
+
+  CefWindowInfo window_info_;
+  CefRefPtr<CefClient> client_;
+  CefBrowserSettings settings_;
+  CefPoint inspect_element_at_;
+};
+
 // Base class for CefBrowserHost implementations. Includes functionality that is
 // shared by the alloy and chrome runtimes. All methods are thread-safe unless
 // otherwise indicated.
@@ -191,6 +208,10 @@ class CefBrowserHostBase : public CefBrowserHost,
   void PrintToPDF(const CefString& path,
                   const CefPdfPrintSettings& settings,
                   CefRefPtr<CefPdfPrintCallback> callback) override;
+  void ShowDevTools(const CefWindowInfo& windowInfo,
+                    CefRefPtr<CefClient> client,
+                    const CefBrowserSettings& settings,
+                    const CefPoint& inspect_element_at) override;
   void ReplaceMisspelling(const CefString& word) override;
   void AddWordToDictionary(const CefString& word) override;
   void SendKeyEvent(const CefKeyEvent& event) override;
@@ -346,6 +367,10 @@ class CefBrowserHostBase : public CefBrowserHost,
   // Called from LoadMainFrameURL to perform the actual navigation.
   virtual bool Navigate(const content::OpenURLParams& params);
 
+  // Called from ShowDevTools to perform the actual show.
+  virtual void ShowDevToolsOnUIThread(
+      std::unique_ptr<CefShowDevToolsParams> params) = 0;
+
   // Create the CefFileDialogManager if it doesn't already exist.
   bool EnsureFileDialogManager();