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Previously, selection and content changes were sent as separate callbacks.
This caused consistency errors in observers, as the two must be processed
simultaneously.

TabStripModelObserver::OnTabStripChanged() carries selection and contents
data at the same time. And it'll be called after tab strip model finishes
it's work. So we don't have to care about model's state that much.

Thus, we can replace following with OnTabStripChanged():
  void TabInsertedAt(...);
  void TabReplacedAt(...);
  void TabMoved(...);
  void TabClosingAt(...);
  void TabDetachedAt(...);
  void TabDeactivated(...);
  void ActiveTabChanged(...);
  void TabSelectionChanged(...);

Also this callback allows us to handle multiple changes in a single
callback. e.g. multiple tab closing, insertion, etc.

Change-Id: Iae824f8be536dbe699ba0108bf242114e0906ec7
Bug: 842194
Reviewed-on: https://chromium-review.googlesource.com/1140105
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579707}