GitLab

Prior to this change, desktop/window capture would emit frames of any
size, regardless of the resolution change policy; and tab capture would
always emit frames of a fixed resolution.  Both cause a problem where
some receivers of the captured video need to do scaling/letterboxing
themselves for proper UX, and other receivers require this be done on
by the capture pipeline.

This change builds upon https://codereview.chromium.org/1124263004,
which allows the client to specify capture constraints that configure
the capture pipeline to do (or not do) the scaling/letterboxing.  All
frame resolution calculation logic has been factored into one place in a
new CaptureResolutionChooser class, which also depends on new utility
functions added to media/base/video_util.*.

BUG=473336

Review URL: https://codereview.chromium.org/1135823004

Cr-Commit-Position: refs/heads/master@{#330232}