Marking flaky https://chromium-review.googlesource.com/c/chromium/src/+/939470

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e95218d0603c4acd09ab2cd361b7953262cf0699

commit e95218d0603c4acd09ab2cd361b7953262cf0699
Author: Henrik Boström <hbos@chromium.org>
Date: Tue Feb 27 15:20:34 2018

Marked canvas-drawImage-live-video.html as flaky on Mac

TBR=zakerinasab@chromium.org

No-Try: True
Bug: 816914
Change-Id: Ib62c7af7241350e80c18412b2c898734fe7b9074
Reviewed-on: https://chromium-review.googlesource.com/939470
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539429}
[modify] https://crrev.com/e95218d0603c4acd09ab2cd361b7953262cf0699/third_party/WebKit/LayoutTests/TestExpectations

According to https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.12?numbuilds=200 it got flaky in the past couple of days. I'll look into it. Degrading to P2 since the test is already disabled on Mac.

The flakiness has first appeared in https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.12/builds/11519.

The flakiness has started after this CL is landed:
https://chromium-review.googlesource.com/c/chromium/src/+/896624

chcunningham@, would you please take a look at this? Thanks.

Sorry, I'm not familiar with Canvas. My only edits to this test are to update the call site of a media util it pulls in.