Add feature flag kDisplaySearchEngineFavicon.

Add the feature flag for displaying search engine favicons in Settings.

Bug: 937057
Change-Id: I2f9ddbbea1adabe8eb6da33e8bce0a40bde34f04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529053
Auto-Submit: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641833}
5 files changed