Image Capture: add a feature flag to ease debugging on the field

This CL adds a feature flag for image capture to ease debugging of
issues in the wild (e.g. https://crbug.com/722038 and
https://crbug.com/709819).  The flag is enabled by default:
  chrome://flags/#enable-image-capture-api

Bug: 735661
Change-Id: Iab06dbc9e6d4a5ecbbf85f70ad7221770915b464
Reviewed-on: https://chromium-review.googlesource.com/544185
Reviewed-by: Alexei Svitkine <[email protected]>
Reviewed-by: Christian Fremerey <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Commit-Queue: Miguel Casas <[email protected]>
Cr-Commit-Position: refs/heads/master@{#481924}
8 files changed