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And puts the implementation in the single place that needs it.
SessionTabHelper is really intended to forward WebContentsObserver
related calls to SessionService. SetTabExtensionAppID() isn't that.
As SetTabExtensionAppID() was called in a single place, I moved
the implementation there.

This is part of a larger refactoring I'm doing in hopes of moving
SessionTabHelper to a shared location.

BUG=1033924
TEST=none

Change-Id: Ia70af33908593839d4a22b4ebbc5f9f48c8abc30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994527
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730200}