vp9: Fix issue in scene/content change detection.

The change https://chromium-review.googlesource.com/#/c/329181/
also changed behavior for cbr mode, which causes some regression
in screenshare test in webrtc.

Resetting the specific change to leave the cbr behavior
unchanged for now.

Change-Id: I52df158806422f86398e1d2f522e92067d8325eb
1 file changed