Teach navigation throttles how to cancel requests in WillProcessResponse.

content/public/browser/navigation_throttle.h

Index: content/public/browser/navigation_throttle.h
diff --git a/content/public/browser/navigation_throttle.h b/content/public/browser/navigation_throttle.h
index fff64b1f4fb8c61f425c27b140786fce5eaa617d..811d7230bd3105e4c26656002f2a60ae8b97a95b 100644
--- a/content/public/browser/navigation_throttle.h
+++ b/content/public/browser/navigation_throttle.h
@@ -55,6 +55,16 @@ class CONTENT_EXPORT NavigationThrottle {
   // CANCEL_AND_IGNORE or DEFER and perform the destruction asynchronously.
   virtual ThrottleCheckResult WillRedirectRequest();
 
+  // Called when a response's headers and metadata are available.
+  //
+  // The implementer is responsible for ensuring that the WebContents this
+  // throttle is associated with remain alive during the duration of this
+  // method. Failing to do so will result in use-after-free bugs. Should the
+  // implementer need to destroy the WebContents, it should return CANCEL,
+  // CANCEL_AND_IGNORE and perform the destruction asynchronously. The

[clamy, 2016/01/25 16:36:21]

Let's remove last condition. There is one known case of a Throttle that will
need to use it (SafeBrowsing on Android).

Also content/public question here for nasko: we're not supposed to land
something in the public API if it's not used outside of content. However,
there's no impl version of the NavigationThrottle, since it's an interface
that's meant to be implemented outside of content. How do we deal with that
case?

[Mike West, 2016/01/26 09:41:42]

Done.

+  // implementer MUST NOT return DEFER.
+  virtual ThrottleCheckResult WillProcessResponse();
+
   // The NavigationHandle that is tracking the information related to this
   // navigation.
   NavigationHandle* navigation_handle() const { return navigation_handle_; }