Clarify resolution change behaviors of video capture devices

There's no functional change in this CL.

I clarified the behavior of resolution changes for video capture devices
by using an enum to describe resolution change policy instead of a
simple boolean.

BUG=388355

Review URL: https://codereview.chromium.org/541163002

Cr-Commit-Position: refs/heads/master@{#294714}
15 files changed