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This CL:

* Implements a WebViewController that automatically adds a close button
  to the nav bar when it's the first VC in the nav stack, and allow
  overriding the right nav button.
* Adds the Credits button to the help center VC.
* Makes the side menu's help button work.

Bug: 740240
Change-Id: I28436ada3ea1a9e588be0cedfc2e5b874c82d4d9
Reviewed-on: https://chromium-review.googlesource.com/572602Reviewed-by: Scott Nichols <nicholss@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487248}