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This CL moves onPageStarted to page commit for browser navigations.
However the old code path is not removed since we still do not want to
enable this for old versions of GMSCore. Add a kill switch feature
that's disabled by default and a test finch config to enable it for
testing.

Bug: 896022
Change-Id: I7cd0777687ca4be096209a3a0cfed8dba4e5bd13
Reviewed-on: https://chromium-review.googlesource.com/c/1327170Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607466}