chromeos: Speculative fix for flaky DragToOverviewTest.DragTab

DragTab fails at about 25% in classic mode under FindIt flake
analysis. It seems that the mouse move to detach the tab sometimes
does not go through when the post event callback is invoked. Add
a WaitForNoPointerHoldLock before sending the mouse move to make
sure the event is handled before its post process callback.

Bug: 942599
Change-Id: Ieea45562b4c8ca7e1ee6bcd9f5cda2a938d1000f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562085Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649673}

chrome/browser/ui/ash/drag_to_overview_interactive_uitest.cc

@@ -201,6 +201,7 @@ IN_PROC_BROWSER_TEST_F(DragToOverviewTest, DragTab) {
   gfx::Vector2d delta(0, drag_length / kSteps);
 
   // Drag tab far enough to detach.
+  test::WaitForNoPointerHoldLock();
   drag_position.Offset(0, GetDetachY(browser_view->tabstrip()));
   ui_controls::SendMouseMoveNotifyWhenDone(
       drag_position.x(), drag_position.y(),