Fully utilize js-test in fast/images/webgl-teximage2d.html to combat flakiness

The test was mixing testPassed()/testFailed() calls with testRunner.waitUntilDone()
and testRunner.notifyDone(), which seemed to result in occasional output
reordering. This patch gets rid of the testRunner calls and instead uses
jsTestIsAsync and finishJSTest().

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/117323007

git-svn-id: svn://svn.chromium.org/blink/trunk@164467 bbb929c8-8fbe-4397-9dbb-9b2b20218538