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* affects system menu, message center, palette bubble, ime bubble, etc.
* WM shadow aperture calculation is adjusted to work better for small
  windows (like the message center when it's empty). There's a TODO
  here which will have to wait for Sebastien to get back so he can give
  feedback on what to do in this case.
* fix alignment of NO_ASSETS bubble --- don't account for
  non-existent stroke.
* existing BubbleBorder tests applied to more different kinds of bubble

BUG=608852

Review-Url: https://codereview.chromium.org/2555373004
Cr-Commit-Position: refs/heads/master@{#438196}