Automated test for dragging between same-page, cross-site frames.

BUG=666858

Review-Url: https://codereview.chromium.org/2549023003
Cr-Commit-Position: refs/heads/master@{#437739}

content/public/test/browser_test_utils.cc

@@ -1327,7 +1327,11 @@ bool DOMMessageQueue::WaitForMessage(std::string* message) {
         new MessageLoopRunner(MessageLoopRunner::QuitMode::IMMEDIATE);
     message_loop_runner_->Run();
   }
-  // The queue should not be empty, unless we were quit because of a timeout.
+  return PopMessage(message);
+}
+
+bool DOMMessageQueue::PopMessage(std::string* message) {
+  DCHECK(message);
   if (message_queue_.empty())
     return false;
   *message = message_queue_.front();

content/public/test/browser_test_utils.h

@@ -442,6 +442,10 @@ class DOMMessageQueue : public NotificationObserver,
   // message. Returns true on success.
   bool WaitForMessage(std::string* message) WARN_UNUSED_RESULT;
 
+  // If there is a message in the queue, then copies it to |message| and returns
+  // true.  Otherwise (if the queue is empty), returns false.
+  bool PopMessage(std::string* message) WARN_UNUSED_RESULT;
+
   // Overridden NotificationObserver methods.
   void Observe(int type,
                const NotificationSource& source,

testing/buildbot/filters/site-per-process.interactive_ui_tests.filter

 # List below tests to be excluded from running.
 
-# This list is currently empty. Hooray!
+# https://crbug.com/666858: No drag-and-drop events should fire...
+-*DragAndDropBrowserTest.CrossSiteDrag*