Rename onGetPixelsEnum back to onGetPixels.

Rename onGetPixelsEnum back to onGetPixels.

Replace the old signature of onGetPixels (return bool) to return an
enum (Result). Remove onGetPixelsEnum.

Add a define for onGetPixelsEnum to onGetPixels. This is for staging
in Chromium, where some implementations override onGetPixelsEnum.

Add the define in skia_for_chromium_defines. Remove
SK_SUPPORT_LEGACY_IMAGE_GENERATOR_RETURN, which is no longer needed by
Chromium.

BUG=skia:3257

Review URL: https://codereview.chromium.org/939113002