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* use TYPE_POPUP for xdg_popup. The differences from TYPE_WINDOW are:
  - no frame
  - ash will not control its bounds
* make popups transient children: this fixes following scenarios
  - z order is correctly updated when parent's z order changes
  - they're grouped in overview.
* implement grab
  - capture even when the grab is requested on the surface
  - transfer capture to child if the child popup requested
     grab.
  - close the surface when capture(grab) is lost.
  - but transfer the grab to the parent if the parent had grab.

BUG=788782
TEST=covered by unit tests. manually tested with gtk3-demo.

Change-Id: I73f5bc7e555f2c28f78dc25a55f77f0a70fd9b78
Reviewed-on: https://chromium-review.googlesource.com/1102354
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568364}