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Previously background web views had timer alignment throttling applied immediately
and budget based throttling after 10s.  It looks like even the timer alignment throttling
on it's own is causing problems for new tabs where the visibility isn't initially known.
To fix this we're changing the policy so we don't do any throttling at all until the web
view has been in the background for 10s and then we turn budget based throttling &
alignment on.

Note this patch removes fast/dom/timer-throttling-hidden-page.html
because the 10s grace period means this test would be too slow.

Virtual time doesn't help because Date.now() (currently) isn't
overridden. There's no support for using a mock time source in
layout tests.  In any event there's plenty of C++ unit test coverage
for this so it's not too bad to remove the layout test.

BUG=649942

Review-Url: https://codereview.chromium.org/2620743002
Cr-Commit-Position: refs/heads/master@{#442989}