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This makes it more clear which WebStateObserver callbacks are called
when. This change does not contain any function changes.

Bug: 903497
Change-Id: Ie774c191ffc2681a7cf1df90089ea25e90a381ac
Reviewed-on: https://chromium-review.googlesource.com/c/1359296Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613265}