Add callbacks for unresponsive render process (fixes #3661)

The client can optionally wait or terminate the render process.

Expose process exit codes via OnRenderProcessTerminated and
CefGetExitCode (fixes #2126).

cefclient: Add a new https://tests/hang page for testing hang behavior.

cefclient: Move message and resource handling to a new BaseClientHandler
class to support loading of test pages in default Chrome UI windows.

libcef/browser/alloy/alloy_browser_host_impl.h

@@ -233,6 +233,13 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
                           const std::string& frame_name,
                           const GURL& target_url,
                           content::WebContents* new_contents) override;
+  void RendererUnresponsive(
+      content::WebContents* source,
+      content::RenderWidgetHost* render_widget_host,
+      base::RepeatingClosure hang_monitor_restarter) override;
+  void RendererResponsive(
+      content::WebContents* source,
+      content::RenderWidgetHost* render_widget_host) override;
   content::JavaScriptDialogManager* GetJavaScriptDialogManager(
       content::WebContents* source) override;
   void RunFileChooser(content::RenderFrameHost* render_frame_host,