If the encoder takes a long time to start up and emit frames the polling
interval of the quality scaler would get out of sync. This causes it to
sometimes make scaling decisions based on only a handful of frames.
This CL ensures that we have observed some minimum number of frames
before deciding to scale up or down.
BUG=b/36734056
Review-Url: https://codereview.webrtc.org/2789483002
Cr-Commit-Position: refs/heads/master@{#17523}
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