Fix incorrect comment about XMLHttpRequest::handleDidCancel()

handleDidCancel() doesn't handle m_loader->cancel() in internalAbort().
It's for handling cancellation coming from other components.

BUG=none

Review URL: https://codereview.chromium.org/467593002

git-svn-id: svn://svn.chromium.org/blink/trunk@180078 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/xml/XMLHttpRequest.h

@@ -211,7 +211,9 @@ private:
     void handleDidFailGeneric();
     // Handles didFail() call not caused by cancellation or timeout.
     void handleNetworkError();
-    // Handles didFail() call triggered by m_loader->cancel().
+    // Handles didFail() call for cancellations. For example, the
+    // ResourceLoader handling the load notifies m_loader of an error
+    // cancellation when the frame containing the XHR navigates away.
     void handleDidCancel();
     // Handles didFail() call for timeout.
     void handleDidTimeout();