Mark a web_test flakily timing out on Fuchsia

http/tests/streams/chromium/get-reader-byob.html is timing out on the Fuchsia
bots some times. Mark it as flaky in TestExpectations.

BUG=1057020

Change-Id: I53f94f98e738682b6ede5efc649cc8e883b27370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078380
Commit-Queue: Adam Rice <ricea@chromium.org>
Auto-Submit: Adam Rice <ricea@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747169}

third_party/blink/web_tests/TestExpectations

@@ -6749,3 +6749,5 @@ crbug.com/1058244 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/s
 
 # Ecosystem-Infra Sheriff 2020-03-04
 crbug.com/1058403 external/wpt/webaudio/the-audio-api/the-audioworklet-interface/suspended-context-messageport.https.html [ Failure ]
+
+crbug.com/1057020 [ Fuchsia ] http/tests/streams/chromium/get-reader-byob.html [ Pass Timeout ]