Do not cancel overview mode if a window drag in progress.

Window activating change might go to cancel overview mode after this
cl: https://chromium-review.googlesource.com/c/chromium/src/+/1338175

Drag a browser window with a bubble dialog ('Restore pages') will
trigger window activation changes from the browser window to the bubble
dialog when drag starts. This will cause cancel overview in
OverviewSession:OnWindowActivating, which will fail the DCHECK that
overview mode should always open during window drag.

This cl makes overview mode will not be cancelled if a window drag in
progress.

Bug: 955814
Change-Id: I77d118cd7e69f58a7a2448d3bfbe1e6310b4f063
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626542
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663320}
2 files changed