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On the way, refactor how the Portal <-> WebContentsImpl pointers are
maintained, as this work caused the raw WebContentsImpl* to dangling while
writing the main code of this CL, which made for a confusing bug. This
refactoring is mainly a mechanical gathering of the code responsible for
updating these into a small nested class.

Bug: 964481
Change-Id: I8a63e1a5531de4807569b35ea0f3fcb19baf343c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976000Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Reviewed-by: Lucas Gadani <lfg@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728057}