Add a way to cancel deferred navigations in NavigationHandle

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 c5202ae71d89d47990394423a87c943af488d66c..fff64b1f4fb8c61f425c27b140786fce5eaa617d 100644
--- a/content/public/browser/navigation_throttle.h
+++ b/content/public/browser/navigation_throttle.h
@@ -17,8 +17,20 @@ class CONTENT_EXPORT NavigationThrottle {
   // This is returned to the NavigationHandle to allow the navigation to
   // proceed, or to cancel it.
   enum ThrottleCheckResult {
+    // The navigation proceeds uninterrupted.
     PROCEED,
+
+    // Defers the navigation until the NavigationThrottle calls
+    // NavigationHandle::Resume or NavigationHandle::CancelDeferredRequest.
+    // If the NavigationHandle is destroyed while the navigation is deferred,
+    // the navigation will be canceled in the network stack.
     DEFER,
+
+    // Cancels the navigation.
+    CANCEL,
+
+    // Cancels the navigation and makes the requester of the navigation acts
+    // like the request was never made.
     CANCEL_AND_IGNORE,
   };
 
@@ -26,9 +38,21 @@ class CONTENT_EXPORT NavigationThrottle {
   virtual ~NavigationThrottle();
 
   // Called when a network request is about to be made for this navigation.
+  //
+  // 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 or DEFER and perform the destruction asynchronously.
   virtual ThrottleCheckResult WillStartRequest();
 
   // Called when a server redirect is received by the navigation.
+  //
+  // 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 or DEFER and perform the destruction asynchronously.
   virtual ThrottleCheckResult WillRedirectRequest();
 
   // The NavigationHandle that is tracking the information related to this