Reland "Inhibit hang watching during views::DesktopDragDropClientWin::StartDragAndDrop"

This is a reland of 40efa001

Original change's description:
> Inhibit hang watching during views::DesktopDragDropClientWin::StartDragAndDrop
> 
> The nested loop process the native events but not the tasks which looks
> like a hang if not inhibited.
> 
> Bug: 806174, 1120381
> Change-Id: Ie298b16a4e0f8cdd6c4ba398d71a7fee02b9d46d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2369253
> Reviewed-by: Robert Liao <robliao@chromium.org>
> Commit-Queue: Robert Liao <robliao@chromium.org>
> Commit-Queue: Oliver Li <olivierli@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#800626}

Bug: 806174
Bug: 1120381
Change-Id: I43fbc7d2d1c1ea2200b8df214389cb40dd65c683
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386198Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803872}

ui/views/widget/desktop_aura/desktop_drag_drop_client_win.cc

@@ -6,6 +6,7 @@
 
 #include <memory>
 
+#include "base/threading/hang_watcher.h"
 #include "ui/base/dragdrop/drag_drop_types.h"
 #include "ui/base/dragdrop/drag_source_win.h"
 #include "ui/base/dragdrop/drop_target_event.h"
@@ -68,6 +69,10 @@ int DesktopDragDropClientWin::StartDragAndDrop(
 
   DWORD effect;
 
+  // Disable hang watching until the end of the function since the user can take
+  // unbounded time to complete the drag. (http://crbug.com/806174)
+  base::HangWatchScopeDisabled disabler;
+
   HRESULT result = ::DoDragDrop(
       ui::OSExchangeDataProviderWin::GetIDataObject(*data.get()),
       drag_source_.Get(),