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This CL enables configuring which trigger types result in a background
fetch. The default configuration can be overridden by a variation
parameter. It also reports the trigger type of each fetch to UMA.

Additionally, this CL enables two additional trigger types that were
previously inactive (Chrome cold start / Chrome foregrounding).

BUG=672479

Review-Url: https://codereview.chromium.org/2626433005
Cr-Commit-Position: refs/heads/master@{#443272}