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Additional changes:
  - Removed NativeWebInterstitialDelegate class
  - Merged HtmlWebInterstitialDelegate to base class
  - Update test to only test WebInterstitial interface

Bug: None
Change-Id: Ic44f309edf8dd85f9a55e45ed731d7abde188039
Reviewed-on: https://chromium-review.googlesource.com/c/1316831
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605423}