Mark fast/canvas/canvas-composite-text-alpha.html flaky

Test case fast/canvas/canvas-composite-text-alpha.html is flaky on Mac,
linux and windows platform. The expected outcome of the test is Pass or
Timeout.

Bug: 1093836
Change-Id: I0f9c8e4ce891cb12ca0be72fd0e506ff3c3b4c68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2249438
Commit-Queue: Yi Xu <yiyix@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779665}

third_party/blink/web_tests/TestExpectations

@@ -6774,3 +6774,4 @@ crbug.com/1095733 http/tests/devtools/tabbed-pane-closeable-persistence.js [ Ski
 # Sheriff 2020-06-17
 crbug.com/1095969 [ Mac ] fast/canvas/OffscreenCanvas-MessageChannel-transfer.html [ Pass Failure ]
 crbug.com/1092975 http/tests/inspector-protocol/target/target-expose-devtools-protocol.js [ Pass Failure ]
+crbug.com/1093836 fast/canvas/canvas-composite-text-alpha.html [ Pass Timeout ]