First failure

https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Linux%20Precise%20Leak/builds/1428
https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux_Precise_Leak/1428/layout-test-results/results.html

says for http/tests/misc/drag-over-iframe-invalid-source-crash.html

({"numberOfLiveActiveDOMObjects":[2,3],"numberOfLiveDocuments":[1,2],"numberOfLiveNodes":[4,23],"numberOfLiveResources":[0,1]})

Suspect:

Delegate dragend to the correct frame's EventHandler.

Currently, most drag-and-drop events are correctly delegated to the
EventHandler of the frame that contains the events' target elements, in
EventHandler::updateDragAndDrop. However,
EventHandler::dragSourceEndedAt does not implement this delegation.  The
issue is visible to the Web in a subtle way -- the frame of the
EventHandler is used to populate the DragEvent's view, which is used to
compute the clientX and clientY attributes, which are user-visible.

This CL implements the delegation used by
MouseEventManager::dispatchDragSrcEvent, into
MouseEventManager::dispatchDragSrcEvent.

BUG= 618770 

Review-Url: https://codereview.chromium.org/2469863003
Cr-Commit-Position: refs/heads/master@{#429669}

I'm not confident enough to revert, but this needs to be disabled.

https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Linux%20Precise%20Leak/ has been red for 16 cycles.

Please investigate!