Use SkBitmapHeap for shaders in SkGPipe.

Use SkBitmapHeap for shaders in SkGPipe.

Required adding a new feature to SkBitmapHeap, allowing it to defer
adding owners, since sometimes we flatten a shader, but then do not
unflatten it, since we already had a copy in the heap, so the owners
never get removed.

Moved to https://codereview.appspot.com/6465047/

[Leon, 2012-08-13 20:26:00]

Depends on resubmitting my earlier change (still looking into the Android
breakage)

[DerekS, 2012-08-14 12:31:55]

lgtm.  The names are starting to get a little confusing, perhaps we could think
about changing up the naming going forward (i.e. in a future CL).  In particular
fSharedHeap is not clear that it only stores bitmaps.