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This CL puts all the public methods together.
It also moves webState, webStateImpl and scrollPosition to private
properties.

Bug: 938255
Change-Id: Ic2b9870da254f538c201893dad87a99f4d18c216
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503323
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638099}