Add UMA metrics for root scroller intervention to track forcing passive breakage.

Calculate the number of event listener invocations that we didn't actually
end up allowing preventDefault to actually execute. The denominator
in this metric is the total number of invocations that we force passive
on.

BUG=625675

Review-Url: https://codereview.chromium.org/2137483003
Cr-Commit-Position: refs/heads/master@{#406002}
17 files changed