Fixing flaky gesture fling test

Removed timeout-based test checks. Instead, the test will make the
assertions and end the test once a number of scroll events have been
received. Hopefully this will fix the flakyness.

BUG=261277

Review URL: https://codereview.chromium.org/124633002

git-svn-id: svn://svn.chromium.org/blink/trunk@164581 bbb929c8-8fbe-4397-9dbb-9b2b20218538