Set testharness.js global timeout to avoid premature test timeouts.

The default testharness.js timeout is only 5 seconds which causes tests
to prematurely timeout when bots are heavily loaded. This causes the tests
to appear flaky. This change sets the testharness.js global timeout to a
value larger than the standard LayoutTest timeout so that the LayoutTest
runner timeout actually controls when the test is considered to have
timed out. This should eliminate the flakiness seen in the mediasource-*
tests.

Review URL: https://chromiumcodereview.appspot.com/22449002

git-svn-id: svn://svn.chromium.org/blink/trunk@155625 bbb929c8-8fbe-4397-9dbb-9b2b20218538