[mac-wk1] LayoutTest webgl/webgl-backing-store-size-update.html is a flaky timeout https://build.webkit.org/results/Apple%20Yosemite%20Release%20WK1%20(Tests)/r201882%20(15312)/results.html https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=webgl%2Fwebgl-backing-store-size-update.html
Test was added with <http://trac.webkit.org/199536>
Marked test as flaky on mac-wk1 in <http://trac.webkit.org/projects/webkit/changeset/201886>