Introduce AncestorThrottle, which will process 'X-Frame-Options' headers.

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 5876343db0decd9bb6933f6243299aa81b825d06..2a62417066d14ef53a0438f3405c6d7fa7ba1095 100644
--- a/content/public/browser/navigation_throttle.h
+++ b/content/public/browser/navigation_throttle.h
@@ -32,6 +32,11 @@ class CONTENT_EXPORT NavigationThrottle {
     // Cancels the navigation and makes the requester of the navigation acts
     // like the request was never made.
     CANCEL_AND_IGNORE,
+
+    // Blocks a navigation due to rules asserted by a response (for instance,
+    // embedding restrictions like 'X-Frame-Options'). This result will only
+    // be returned from WillProcessResponse.

[carlosk, 2016/05/10 14:05:36]

Should also mention that BLOCK_RESPONSE should only be used for subframes (for
now at least, as stated in navigation_resource_throttle.cc).

+    BLOCK_RESPONSE,
   };
 
   NavigationThrottle(NavigationHandle* navigation_handle);
@@ -61,7 +66,8 @@ class CONTENT_EXPORT NavigationThrottle {
   // 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.
+  // CANCEL_AND_IGNORE, or BLOCK_RESPONSE and perform the destruction
+  // asynchronously.
   virtual ThrottleCheckResult WillProcessResponse();
 
   // The NavigationHandle that is tracking the information related to this