Remove flaky suppression for fast/webgl/webgl-composite-modes.html

This test is no longer flaky.

Bug: 846547
Change-Id: Ia8062aa23eca080a7904a6ced767ffff888ff39f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2029493
Auto-Submit: Austin Eng <enga@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737029}

third_party/blink/web_tests/TestExpectations

@@ -5095,7 +5095,6 @@ crbug.com/836783 fast/peerconnection/RTCRtpSender-setParameters.html [ Pass Time
 crbug.com/845610 [ Win ] http/tests/inspector-protocol/target/target-browser-context.js [ Pass Failure ]
 
 # Sheriff 2018-05-25
-crbug.com/846547 fast/webgl/webgl-composite-modes.html [ Pass Failure ]
 crbug.com/846656 external/wpt/css/selectors/focus-visible-002-manual.html [ Pass Timeout ]
 
 # Sheriff 2018-05-28