If an mp4 video is requested via <video src=url>, then the request should have "Chrome-Proxy-Accept-Transform: compressed-video" and the response should have:
  - Content-Type: video/webm
  - Chrome-Proxy-Content-Transform: compressed-video
  - Via and XOCL as usual

Not true for XHR

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9714965fa2df69c6b6c21171a193cb25027cc186

commit 9714965fa2df69c6b6c21171a193cb25027cc186
Author: robertogden <robertogden@chromium.org>
Date: Fri Feb 03 19:39:33 2017

Add test to check for no compression for XHR video.

Video requests made by XHR should not be proxied.

BUG= 688069 

Review-Url: https://codereview.chromium.org/2676673004
Cr-Commit-Position: refs/heads/master@{#448052}

[modify] https://crrev.com/9714965fa2df69c6b6c21171a193cb25027cc186/tools/chrome_proxy/webdriver/video.py