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It seems that ChromeExtensionsClient::Init() is taking a non-trivial
amount of time (see linked bug). Add UMA so that we can track it,
and measure any future improvements.

Bug: 847237
Change-Id: I1b86dd9a8f2a7500841f253580a9e6d64c9f4d45
Reviewed-on: https://chromium-review.googlesource.com/1142617Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577700}