Implement NavigationThrottle::BLOCK_REQUEST_AND_COLLAPSE.

content/public/browser/navigation_handle.h

Index: content/public/browser/navigation_handle.h
diff --git a/content/public/browser/navigation_handle.h b/content/public/browser/navigation_handle.h
index 684fd26aa145b62a67505afb0e6db0e3a5ab4456..fecc77f71fc1e8adbd88874f6df2fa817aab8268 100644
--- a/content/public/browser/navigation_handle.h
+++ b/content/public/browser/navigation_handle.h
@@ -233,8 +233,10 @@ class CONTENT_EXPORT NavigationHandle {
   virtual void Resume() = 0;
 
   // Cancels a navigation that was previously deferred by a NavigationThrottle.
-  // |result| should be equal to NavigationThrottle::CANCEL or
-  // NavigationThrottle::CANCEL_AND_IGNORE.
+  // |result| should be equal to either:
+  //  - NavigationThrottle::CANCEL,
+  //  - NavigationThrottle::CANCEL_AND_IGNORE. or
+  //  - NavigationThrottle::BLOCK_REQUEST_AND_COLLAPSE.

[clamy, 2017/04/24 13:15:18]

I believe we can cancel the Navigation with more values than those. This should
rather be:
|result| should not be equal to NavigationThrottle::PROCEED or
NavigationThrottle::DEFER.

[engedy, 2017/04/28 13:43:59]

There is a DCHECK in [3] that used to allow only the two CANCEL's, and now I
added B_R_A_C to that list, hence the comment.

[3]:
https://cs.chromium.org/chromium/src/content/browser/frame_host/navigation_ha...

   // Note: this may lead to the deletion of the NavigationHandle and its
   // associated NavigationThrottles.
   virtual void CancelDeferredNavigation(