DragController: Set ExecutionContext when DataTransfer is populated

This allows for clearing the DragState in case the Document is scheduled
for shutdown.

Bug: 928781, 843903
Change-Id: Ie00428a7503122c68a0e96e32d124301b8b381f8
Reviewed-on: https://chromium-review.googlesource.com/c/1485834Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635073}

third_party/blink/renderer/core/page/drag_controller.cc

@@ -995,6 +995,11 @@ bool DragController::PopulateDragDataTransfer(LocalFrame* src,
     // FIXME: For DHTML/draggable element drags, write element markup to
     // clipboard.
   }
+
+  // Observe context related to source to allow dropping drag_state_ when the
+  // Document goes away.
+  SetContext(src->GetDocument());
+
   return true;
 }