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. The sample WindowManager creates various containers, and marks the container
  for user windows to be allowed to have active windows. Other containers may
  also be allowed to have active children in the future.

. For mandoline, the BrowserWindow (which is the WM) marks the root as being
  able to contain active windows. Consequently, the browser window can be
  properly activated.

. Raise a Window to the top when it becomes active.

BUG=548422

Review URL: https://codereview.chromium.org/1459463004

Cr-Commit-Position: refs/heads/master@{#361274}