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There are two options to implement a callback for title changes.
* Implement callback for when the active nav entry changes, and a
  callback for when the title of the nav entry changes
* A callback for when the title of a tab changes, ignoring details about
  things like navigations.

This CL takes the second option, since it's both simpler to implement
and simpler to use.

Bug: 1060997
Change-Id: I5ed1a3e41abcf5f7d7cf3c65cbd94533e8d9c36f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2115695
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753490}