This is mostly a reland of
https://chromium-review.googlesource.com/c/chromium/src/+/807604
with some improvements and now unblocked by changes to bots.

Media layout tests need repair
1) Many tests check for proprietary codec support by only asking about
   "video/mp4" without specifying a codec. This check is busted
   because MP4 is no longer considered proprietary while codecs like
   H264 and AAC still are. We currently claim "video/mp4" is entirely
   unsupported via test hacks to avoid breaking those checks.
2) Some MSE tests focus on MP4 with H264. While many bots now have the
   needed codecs, we artificially remove their support via test-only
   hacks in order to keep expectations the same for all runners.

This CL does the following
1) Remove incomplete checks for "video/mp4" - instead hardcode use of
   the .ogv files (preserving the existing behavior).
2) Update MSE mp4/h264 test expectations to PASS. Starting this week
   all webkit bots include the needed codecs.
3) Remove test hacks that artificially reduce codec support.

It is also not a goal to make LayoutTests try all the supported codecs.
That is covered by unit/integration tests.

Due to 3) above, some external/wpt/media-source tests now fail because
they select mp4. Fixing these failures is tracked in Issue 794338.

TBR: [email protected], [email protected]
Bug: 327115,746579,787575,568704,794338
Change-Id: I8443d32fc9871c887d0009605cf8876521844916
Reviewed-on: https://chromium-review.googlesource.com/827649
Reviewed-by: Chrome Cunningham <[email protected]>
Commit-Queue: Chrome Cunningham <[email protected]>
Cr-Commit-Position: refs/heads/master@{#525212}