Add Page.stopLoading to devtools and --timeout switch to headless_shell

headless/public/headless_web_contents.h

Index: headless/public/headless_web_contents.h
diff --git a/headless/public/headless_web_contents.h b/headless/public/headless_web_contents.h
index 74dc4df9049d83f4bdc1c8fd42ba83ce736d5cec..85e642592a2737b579d5a9330b5e7640024f9295 100644
--- a/headless/public/headless_web_contents.h
+++ b/headless/public/headless_web_contents.h
@@ -71,6 +71,9 @@ class HEADLESS_EXPORT HeadlessWebContents {
   // Close this page. |HeadlessWebContents| object will be destroyed.
   virtual void Close() = 0;
 
+  // Cancels any pending navigation.
+  virtual void Stop() = 0;
+
  private:
   friend class HeadlessWebContentsImpl;
   HeadlessWebContents() {}