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This change will allow us to measure image capture constraints usage and
to decide if we can put the existing "zoom" constraint behind a
permission prompt without much difficulty.

Test: Go to beaufortfrancois.github.io/sandbox/image-capture/playground
and check chrome://histograms/Blink.UseCounter.Features is updated with
requested constraints.

Bug: 934063
Change-Id: I4f5bcb6fc2b9b0c860127c4167fd2258fbaee975
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151586Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Rijubrata Bhaumik <rijubrata.bhaumik@intel.com>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#760216}