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/alloy/alloy_browser_host_impl.h

@@ -85,10 +85,6 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
             bool matchCase,
             bool findNext) override;
   void StopFinding(bool clearSelection) override;
-  void ShowDevTools(const CefWindowInfo& windowInfo,
-                    CefRefPtr<CefClient> client,
-                    const CefBrowserSettings& settings,
-                    const CefPoint& inspect_element_at) override;
   void CloseDevTools() override;
   bool HasDevTools() override;
   bool IsWindowRenderingDisabled() override;
@@ -127,6 +123,9 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
                             const CefSize& max_size) override;
   CefRefPtr<CefExtension> GetExtension() override;
   bool IsBackgroundHost() override;
+  bool CanExecuteChromeCommand(int command_id) override;
+  void ExecuteChromeCommand(int command_id,
+                            cef_window_open_disposition_t disposition) override;
 
   // Returns true if windowless rendering is enabled.
   bool IsWindowless() const override;
@@ -285,6 +284,10 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
       override;
   void WebContentsDestroyed() override;
 
+ protected:
+  void ShowDevToolsOnUIThread(
+      std::unique_ptr<CefShowDevToolsParams> params) override;
+
  private:
   friend class CefBrowserPlatformDelegateAlloy;