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This CL adds pin/unpin metrics when user clicks pin/unpin button.

Bug: 1136254
Change-Id: I9bbbac6444cbbf2d74d74abbd8ec7d8b7ec31a4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462195
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815484}