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.cc

@@ -18,6 +18,7 @@
 #include "libcef/browser/browser_platform_delegate.h"
 #include "libcef/browser/context.h"
 #include "libcef/browser/devtools/devtools_manager.h"
+#include "libcef/browser/hang_monitor.h"
 #include "libcef/browser/media_access_query.h"
 #include "libcef/browser/osr/osr_util.h"
 #include "libcef/browser/request_context_impl.h"
@@ -1196,6 +1197,20 @@ void AlloyBrowserHostImpl::WebContentsCreated(
                      std::move(platform_delegate), /*extension=*/nullptr);
 }
 
+void AlloyBrowserHostImpl::RendererUnresponsive(
+    content::WebContents* source,
+    content::RenderWidgetHost* render_widget_host,
+    base::RepeatingClosure hang_monitor_restarter) {
+  hang_monitor::RendererUnresponsive(this, render_widget_host,
+                                     hang_monitor_restarter);
+}
+
+void AlloyBrowserHostImpl::RendererResponsive(
+    content::WebContents* source,
+    content::RenderWidgetHost* render_widget_host) {
+  hang_monitor::RendererResponsive(this, render_widget_host);
+}
+
 content::JavaScriptDialogManager*
 AlloyBrowserHostImpl::GetJavaScriptDialogManager(content::WebContents* source) {
   if (!javascript_dialog_manager_) {