[Image Fetcher] Add feature-specific metrics reporting

Adds a required client name for ImageFetcher calls. The client name
is used to report to client-specific histograms. These will be used
to determine how much each feature contributes to:
- Disk usage
- Cache utilization
- Fetch/decode times

These factors will be used to make informed decisions about which
features benefit from caching.

Bug: 916722
Change-Id: I22101ebc973eb5f58e8794345065b1983c95b2d7
Reviewed-on: https://chromium-review.googlesource.com/c/1393671
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631014}
35 files changed