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changes:
- WindowState caches a member for saving PersistentWindowInfo, which is
  saved on OnWillProcessDisplayChanges() and restored on
  OnDisplayConfigurationChanged().
- Saving when it hasn't been saved.
- Restoring when (1) we have saved persistent info, (2) persistent
  display id is a valid display currently, (3) persistent display id
  is not current display id. We could have display position/layout
  changes, in which case we transform the window accordingly.
- Reset saved PersistentWindowInfo each time window bounds have restored
  or when bounds changed by user.

Bug: 805046
Test: ash_unittests --gtest_filter=PersistentWindowControllerTest*
Change-Id: I67426ca17f0d25f92284eb690f849bf77bb748be
Reviewed-on: https://chromium-review.googlesource.com/893644Reviewed-by: Mitsuru Oshima (In Tokyo) <oshima@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Qiang Xu <warx@google.com>
Cr-Commit-Position: refs/heads/master@{#537849}