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This modernizes the WebContentsObserver interface.

BUG=767526

Change-Id: I55b92fabf7a62cc932124822a91da1ffe2230549
Reviewed-on: https://chromium-review.googlesource.com/889939Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532563}