OptimizationGuide: Make the cmd line override flag blocking

This helps with manual testing and automation where Chrome opens a page
at startup.

Bug: 919933
Change-Id: Ie3df69c91584a32c8b8d5287c18614b5de5538fc
Reviewed-on: https://chromium-review.googlesource.com/c/1407778
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622635}