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Now that SkImageGenerator is hooked up to use SkCodec by default,
we need to compile SkCodec in order to compile Skia.

The good news is that we can now turn on/off individual codecs.

This CL enables SkCodec on iOS, but does not turn on any of the
codecs that require third_party libraries.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1710493002

Review URL: https://codereview.chromium.org/1710493002