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The referenced_frame_ids set by Vp8Encoder were wrong in two ways:
1) A recent change introduced an off-by-one error.  2) Math was being
done with results stored in a uint8 variable, which was causing the
upper 24 bits to be stripped.

Added VideoEncoderImplTest.GeneratesKeyFrameThenOnlyDeltaFrames test to
prevent future regressions.  In addition, a lot of unit test code clean-up
was done to beef-up existing testing while also removing useless tests.

Review URL: https://codereview.chromium.org/581903002

Cr-Commit-Position: refs/heads/master@{#295493}