Modify the frame flags to update the ARF on top layer,
for the tests:
VP9/DatarateTestVP9Large.BasicRateTargeting3TemporalLayers
VP9/DatarateTestVP9Large.BasicRateTargeting3TemporalLayersFrameDropping

This is needed to fix the encode/decoder mismatches caused by 863f860b,
and removed in the revert e9b7f98c.

Change-Id: I6b9fecfdd17315fc0179e29949338c77636026c0