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 - Use element.animate to resize the overlay for embedded extension
   options. When an <extensionoptions> element is embedded in a WebUI
   overlay, if its size changes the overlay will smoothly expand to
   surround the element.
 - Add an API to <extensionoptions> to defer autosizing until its
   embedder is ready. This is used to 'pin' the element's size until
   the overlay is done expanding.
 - Move the <extensionoptions> off screen until the overlay is done
   expanding.
 - Prevent <extensionoptions> from shrinking after the initial sizing.
   This makes the look of the embedded options page more consistent.

BUG=386842

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291286

Review URL: https://codereview.chromium.org/480243003

Cr-Commit-Position: refs/heads/master@{#291583}