Report repetition count in SkCodec

Report repetition count in SkCodec

Add a new accessor to retrieve the repetition count.

Remove constants (and corresponding copyright) in SkCodecAnimation.
These may make sense for the calling code, but are not needed here.

kRepetitionCountInfinite corresponds to Blink's kAnimationLoopInfinite.
Move cLoopCountNotSeen to private. It is used to determine whether we
still need to parse. Add a new enum to the parse query - only parse
enough to determine the repetition count.

Unlike Chromium, SkGifCodec does not account for deleting the reader
(which SkGifCodec does not do) or failed decodes.

Add a test.

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Review-Url: https://codereview.chromium.org/2447863002