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.

patch/patches/views_widget.patch

@@ -414,7 +414,7 @@ index 2f552f72074e3..3f057242d198c 100644
  }
  
 diff --git ui/views/widget/widget.h ui/views/widget/widget.h
-index 419b75d0608b2..d772dc17d165c 100644
+index 419b75d0608b2..57bb1fca770ed 100644
 --- ui/views/widget/widget.h
 +++ ui/views/widget/widget.h
 @@ -351,6 +351,8 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
@@ -426,6 +426,15 @@ index 419b75d0608b2..d772dc17d165c 100644
      // Specifies the initial bounds of the Widget. Default is empty, which means
      // the NativeWidget may specify a default size. If the parent is specified,
      // |bounds| is in the parent's coordinate system. If the parent is not
+@@ -742,7 +744,7 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
+   void ShowInactive();
+ 
+   // Activates the widget, assuming it already exists and is visible.
+-  void Activate();
++  virtual void Activate();
+ 
+   // Deactivates the widget, making the next window in the Z order the active
+   // window.
 diff --git ui/views/widget/widget_delegate.h ui/views/widget/widget_delegate.h
 index 6d87be86ae8b3..02fe7a11958d0 100644
 --- ui/views/widget/widget_delegate.h