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When taking photo, there are two stages:
1. Click shutter button -> Shutter is done
2. Shutter is done -> Captured image is returned and save

We already have 'photo-taking' event to measure (1) + (2) and
'photo-capture-post-processing' for (2). It will be helpful to have a
'photo-capture-shutter' for (1) as well.

In addition, to measure performance for front camera and back camera
respectively, we can add events with facing information in their names.
Therefore, the ones without facing information are the average metrics
and the ones with facing information are results for specific facing.

Bug: b/149454553
Test: tast run [DUT] camera.CCAUIPerf
Change-Id: I5889ddde4df7c1b3ca4a71ba1c1c0f1628af2fff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087501Reviewed-by: Shik Chen <shik@chromium.org>
Reviewed-by: Kuo Jen Wei <inker@chromium.org>
Commit-Queue: Wei Lee <wtlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749134}