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Presently, BubbleDialogDelegate is a WidgetObserver, and observes both
its anchor widget and its own widget for various events. There is a
problem with this: subclasses are unintentionally abusing the fact that
BubbleDialogDelegate is a WidgetObserver to observe changes to either
the anchor widget or the bubble widget. Very few of these subclasses:

* Are aware that BubbleDialogDelegate actually observes multiple
  widgets
* Are aware of the intricacies of when BubbleDialogDelegate starts &
  stops observing widgets
* Bother to call the BubbleDialogDelegate implementations of the
  WidgetObserver methods from their overrides

Change-Id: I9de0d1429431f33e80b564ad3e545c976d141a7e
Bug: 1011446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227054
Commit-Queue: Robert Liao <robliao@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774700}