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.

include/cef_request_handler.h

@@ -48,6 +48,7 @@
 #include "include/cef_request.h"
 #include "include/cef_resource_request_handler.h"
 #include "include/cef_ssl_info.h"
+#include "include/cef_unresponsive_process_callback.h"
 #include "include/cef_x509_certificate.h"
 
 ///
@@ -221,14 +222,52 @@ class CefRequestHandler : public virtual CefBaseRefCounted {
   /*--cef()--*/
   virtual void OnRenderViewReady(CefRefPtr<CefBrowser> browser) {}
 
+  ///
+  /// Called on the browser process UI thread when the render process is
+  /// unresponsive as indicated by a lack of input event processing for at
+  /// least 15 seconds. Return false for the default behavior which is an
+  /// indefinite wait with the Alloy runtime or display of the "Page
+  /// unresponsive" dialog with the Chrome runtime. Return true and don't
+  /// execute the callback for an indefinite wait without display of the Chrome
+  /// runtime dialog. Return true and call CefUnresponsiveProcessCallback::Wait
+  /// either in this method or at a later time to reset the wait timer,
+  /// potentially triggering another call to this method if the process remains
+  /// unresponsive. Return true and call CefUnresponsiveProcessCallback::
+  /// Terminate either in this method or at a later time to terminate the
+  /// unresponsive process, resulting in a call to OnRenderProcessTerminated.
+  /// OnRenderProcessResponsive will be called if the process becomes responsive
+  /// after this method is called. This functionality depends on the hang
+  /// monitor which can be disabled by passing the `--disable-hang-monitor`
+  /// command-line flag.
+  ///
+  /*--cef()--*/
+  virtual bool OnRenderProcessUnresponsive(
+      CefRefPtr<CefBrowser> browser,
+      CefRefPtr<CefUnresponsiveProcessCallback> callback) {
+    return false;
+  }
+
+  ///
+  /// Called on the browser process UI thread when the render process becomes
+  /// responsive after previously being unresponsive. See documentation on
+  /// OnRenderProcessUnresponsive.
+  ///
+  /*--cef()--*/
+  virtual void OnRenderProcessResponsive(CefRefPtr<CefBrowser> browser) {}
+
   ///
   /// Called on the browser process UI thread when the render process
-  /// terminates unexpectedly. |status| indicates how the process
-  /// terminated.
+  /// terminates unexpectedly. |status| indicates how the process terminated.
+  /// |error_code| and |error_string| represent the error that would be
+  /// displayed in Chrome's "Aw, Snap!" view. Possible |error_code| values
+  /// include cef_resultcode_t non-normal exit values and platform-specific
+  /// crash values (for example, a Posix signal or Windows hardware exception).
   ///
   /*--cef()--*/
   virtual void OnRenderProcessTerminated(CefRefPtr<CefBrowser> browser,
-                                         TerminationStatus status) {}
+                                         TerminationStatus status,
+                                         int error_code,
+                                         const CefString& error_string) {}
 
   ///
   /// Called on the browser process UI thread when the window.document object of