Fix a broken WebGL layout test by using testRunner.displayAsyncThen().

The expected result of fast/canvas/webgl/draw-webgl-to-canvas-2d.html
is failure because this test uses setTimeout to go to next frame.
testRunner.displayAsyncThen() can fix this test.

BUG=423674

Review URL: https://codereview.chromium.org/655133003

git-svn-id: svn://svn.chromium.org/blink/trunk@183817 bbb929c8-8fbe-4397-9dbb-9b2b20218538