Fix appcache/fallback test race exposed by the Blink Scheduler.

The blink scheduler exposed a race where the onnoupdate event could happen
before the iframe has been loaded, causing the test to become flaky. This
fix replaces the use of onnoupdate with an onload event on the iframe.

Also remove linux specific expectation since the linux expected result is
now the same as all other platforms.

BUG=432129

Review URL: https://codereview.chromium.org/720963002

git-svn-id: svn://svn.chromium.org/blink/trunk@185279 bbb929c8-8fbe-4397-9dbb-9b2b20218538