[AW] Add Variations.RestartsWithStaleSeed metric.

This metric records the number of consecutive times the WebView browser
process starts with a stale seed. It's written when a fresh seed is
loaded after previously loading a stale seed.

Test: Manually verified with logging that the metric was written.
Bug: 1010625
Change-Id: Iadedb45af08d59ecd6662472670f848e8e99a8d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1851126
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709417}