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In recent CLs that have turned on webview-based signin, the old chrome
signin url has been adopted by the new signin mechanism, but this has
resulted in the webview's embedder being placed in a separate storage
partition, which has broken both default page zoom behaviour.
It has also prevented the appearance of the signin
page's zoom level in the zoom exceptions list in
settings.

This CL restores previous behaviour, where the webview's embedder lives
in the default storage partition.

BUG=462559

Review URL: https://codereview.chromium.org/996743003

Cr-Commit-Position: refs/heads/master@{#322050}