change back to method for localmatrix imagefilter

This method is different from MatrixFilter, in that MatrixFilter does not require a pre-existing
filter, but LocalM does. Also change the comment to be more general, as there is no promise that
we return a different subclass, and certainly not a specific subclass.

This pattern of obj->newWithModifiers() also more closely matches the pattern in SkImage (newSubset).

BUG=skia:

Review URL: https://codereview.chromium.org/1402133002
3 files changed