The root cause is that Cadence method essentially takes
local average of frame durations, and then in Variable
Frame Rate case, long frame would be renderered shorter
than it should be, and short frame would be renderered
longer than it should be.

In this CL, we add a variable frame rate detection
mechanism, and disable Cadence if variable frame rate gets
detected.

BUG=555917

Review URL: https://codereview.chromium.org/1459923003

Cr-Commit-Position: refs/heads/master@{#362737}