Image loading tests have become flaky after #3052

[kenrussell]

#3052 seems to have introduced significant flakiness into the tests which load cross-origin images. On Chromium's automated testing system, the following bugs have been found:

WebGL origin-clean-conformance-* test fail because image loading failed
http://crbug.com/1082525

conformance/textures/misc/origin-clean-conformance.html is flaky
http://crbug.com/1083100

@jdashg @shrekshao @jdarpinian how can we fix this flakiness? In hindsight, 1000 ms is too short a timeout for some slow bots.

[kenrussell]

One more bug:

conformance/more/functions/readPixelsBadArgs.html is flaky
http://crbug.com/1083320

[kdashg]

Make the delay bigger I guess.