Speculative attempt to unflake preload iframe test

https://codereview.chromium.org/1738133002 landed an iframe test for preload,
which seems to be slightly flaky. This is an attempt to unflake the test
by increasing its timeout.

BUG=597097

Review URL: https://codereview.chromium.org/1827773002

Cr-Commit-Position: refs/heads/master@{#383077}
1 file changed