Remove kCustomSearchEngines flag.

The feature "Custom Search Engines" has been launched in M72, so the
feature flag kCustomSearchEngines can be removed now.

Bug: 433824
Change-Id: I213767cdc4a4e48b95f27da79385d8b0be45caee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1528630
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641832}
9 files changed