DecodingImageGenerator implmentation of SkImageGenerator

DecodingImageGenerator implmentation of SkImageGenerator

Add new DecodingImageGenerator class, which is Blink's implementation
of the Skia SkImageGenerator interface.  This is an interface that
allows a SkDiscardablePixelRef to re-decode an image.

This is only a proof-of-concept for the SkImageGenerator interface.

This depends on <https://codereview.chromium.org/74793011/>;

TODO:

-   The interface between the DecodingImageGenerator and the
    ImageFrameGenerator objects needs to be improved to reduce memory
    copying.

-   Use this class in the DeferredImageDecoder:
    SkBitmap DeferredImageDecoder::createLazyDecodingBitmap(size_t index)
    {
        SkBitmap bitmap;
        (void)DecodingImageGenerator::Install(m_frameGenerator, index, &bitmap);
        return bitmap;
    }

BUG=