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This adds a way to execute javascript in WebLayer. The script will be
executed in a weblayer isolate that will not be able to access any
scripts from the page.

Change-Id: Ia17bd8476b5d9247d5761f0af24cd4d61edaccb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867300
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707455}