Mark some media tests as flaky that fail on racy first frame paint

There are some media tests that occasionally flake on many different
bots. They all have the same underlying issue that sometimes the first
frame of the video hasn't painted yet when the screenshot is taken.
Marking them as flaky for now until we can figure out a deterministic
way to make them pass.

Example failure:
https://ci.chromium.org/p/chromium/builders/ci/Mac10.10%20Tests/48598

Bug: 1024976
Change-Id: I66e970c499f6719adcfd400966cf225aced68723
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918321
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Auto-Submit: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715861}

third_party/blink/web_tests/TestExpectations

@@ -5807,6 +5807,14 @@ external/wpt/trusted-types/eval-no-csp-no-tt-default-policy.tentative.html [ Fai
 # Temporarily disabled to change the API contract in V8
 crbug.com/1018029 virtual/wasm-site-isolated-code-cache/http/tests/devtools/wasm-isolated-code-cache/wasm-cache-test.js [ Pass Failure ]
 
+# First frame not always painted in time
+crbug.com/1024976 media/controls/paint-controls-webkit-appearance-none.html [ Pass Failure ]
+crbug.com/1024976 media/controls/paint-controls-webkit-appearance-none-custom-bg.html [ Pass Failure ]
+crbug.com/1024976 media/video-no-audio.html [ Pass Failure ]
+crbug.com/1024976 virtual/audio-service/media/controls/paint-controls-webkit-appearance-none.html [ Pass Failure ]
+crbug.com/1024976 virtual/audio-service/media/controls/paint-controls-webkit-appearance-none-custom-bg.html [ Pass Failure ]
+crbug.com/1024976 virtual/audio-service/media/video-no-audio.html [ Pass Failure ]
+
 # Sheriff 2019-10-31
 crbug.com/1020036 [ Debug ] external/wpt/lifecycle/freeze.html [ Failure Pass ]