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Replace old API with new API. In order to do this, changed
the API a little bit.

* Notify before deleting web conents when tabs are closed.
* Pass TabStripModel* in OnTabStripModelChanged().

This CL has no intended behavior change. Reordered methods
definitions so that they match with declarartion order.

Change-Id: I704cfb3418d06a7539096307aefab916338c65e9
Bug: 842194
Reviewed-on: https://chromium-review.googlesource.com/1171963
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582840}