GitLab

Add SearchEngineTabHelper without any functionality. This TabHelper will
be the main worker for custom search engines feature.

Bug: 433824
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ie18b9237d0f1df52d94ff2c9b4e70b299ab426b1
Reviewed-on: https://chromium-review.googlesource.com/c/1288431
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600759}