Register some flattenable factories.

Register some flattenable factories.

Now, SkComposeShaders, SkDiscreteMapper, SkCosineMapper,
Line2DPathEffect, and SkTransparentShader can be flattened
and unflattened. This makes more of our samples work when
using a pipe.

[Leon, 2012-06-25 20:15:58]

The others may be less important, but I assume SkComposeShader should work since
it's in core? Should I create a GM for that? (We already have a sample; it could
be fairly easily converted to a GM.)

[reed1, 2012-06-25 20:21:38]

lgtm