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This partially reverts commit c878198d.
The programmatic computation is only enabled for extension and
autogenerated themes for now.  Long-term, we will figure out something
we can do consistently, then enable that for all themes.

Bug: 878664, 1068907
Change-Id: I8e66aac3f75dd25f0938d8ff1a61b1add4b855f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2141650
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757619}