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Add and use an extension_install.proto file. This will allow us to
gather more detailed metrics on the types of extensions that users
have installed, and determine the answers to more structured queries
than could be determined by UMA histograms alone.

Add the extension_install.proto and an ExtensionInstallMetricsProvider
to gather these metrics, and hook these up to the metrics service.

Add unittests for the provider.

Bug: 673875
Change-Id: Ic0bdded27cafe3de75f2edeec2e7d927e0e2ecb6
Reviewed-on: https://chromium-review.googlesource.com/621672
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505259}