SkCodec no longer inherits from SkImageGenerator.

SkCodec no longer inherits from SkImageGenerator.

SkImageGenerator makes some assumptions that are not necessarily valid
for SkCodec. For example, SkCodec does not assume that it can always be
rewound.

We also have an ongoing question of what an SkCodec should report as
its default settings (i.e. the return from getInfo). It makes sense for
an SkCodec to report that its pixels are unpremultiplied, if that is
the case for the underlying data, but if a client of SkImageGenerator
uses the default settings (as many do), they will receive
unpremultiplied pixels which cannot (currently) be drawn with Skia. We
may ultimately decide to revisit SkCodec reporting an SkImageInfo, but
I have left it unchanged for now.

Import features of SkImageGenerator used by SkCodec into SkCodec.

I have left SkImageGenerator unchanged for now, but it no longer needs
Result or Options. This will require changes to Chromium.

Manually handle the lifetime of fScanlineDecoder, so SkScanlineDecoder.h
can include SkCodec.h (where Result is), and SkCodec.h does not need
to include it (to delete fScanlineDecoder).

In many places, make the following simple changes:
- Now include SkScanlineDecoder.h, which is no longer included by
  SkCodec.h
- Use the enums in SkCodec, rather than SkImageGenerator
- Stop including SkImageGenerator.h where no longer needed

Review URL: https://codereview.chromium.org/1220733013