Support "Back to tab" for Chrome app windows

This CL makes sure ActivateContents() is implemented for Chrome App
windows so that Back to Tab button in the Picture-in-Picture window
focuses back the window.

Bug: 938250
Change-Id: I4814c52553b8b938bff4005f032a3f83f2e54ce8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503255Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#638030}

extensions/browser/app_window/app_window.cc

@@ -881,6 +881,10 @@ void AppWindow::UpdateNativeAlwaysOnTop() {
   }
 }
 
+void AppWindow::ActivateContents(WebContents* contents) {
+  native_app_window_->Activate();
+}
+
 void AppWindow::CloseContents(WebContents* contents) {
   native_app_window_->Close();
 }

extensions/browser/app_window/app_window.h

@@ -391,6 +391,7 @@ class AppWindow : public content::WebContentsDelegate,
   friend class PlatformAppBrowserTest;
 
   // content::WebContentsDelegate implementation.
+  void ActivateContents(content::WebContents* contents) override;
   void CloseContents(content::WebContents* contents) override;
   bool ShouldSuppressDialogs(content::WebContents* source) override;
   content::ColorChooser* OpenColorChooser(