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This is a prerequisite change for supporting variable video frame sizes
in Cast.  This allows a follow-up change to remove the width/height
properties from VideoSenderConfig.  Instead, the width/height is passed
to the ctor in a separate gfx::Size argument.

BUG=451277

Review URL: https://codereview.chromium.org/883013003

Cr-Commit-Position: refs/heads/master@{#313779}