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This CL adds the histogram
Extensions.Events.DispatchWithServiceWorkerBackground when an extension
event is dispatched to the background service worker of an extension.

Bug: None
Change-Id: I6a119dfb4d39b518201c8cf166e99a2f4595dac1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441949
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812576}