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This CL extends the expiration of the WebVTT histogram metrics, which
will be useful gauging the usage of WebVTT on the web and prioritizing
future investments in the area.

Bug: 1140006
Change-Id: I7d91609685518aed61140535766aa8cf77b16bd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542772Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#830726}