DragController: Clear DragState in DragEnd

DragState is a ScriptWrappable which can keep alive JavaScript upon
unified heap garbage collection resulting in a leak of the last Window
that used a drag event. Clear the state when the drag ended.

Bug: 843903, 928781
Change-Id: Id86a1a917fe772f56fb679e1d73ef3453785ce6d
Reviewed-on: https://chromium-review.googlesource.com/c/1456073Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629582}

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

@@ -215,6 +215,7 @@ void DragController::ClearDragCaret() {
 void DragController::DragEnded() {
   drag_initiator_ = nullptr;
   did_initiate_drag_ = false;
+  drag_state_ = nullptr;
   page_->GetDragCaret().Clear();
 }