Warn if return values from SkRect's Make* methods are ignored

Warn if return value from SkRect's Make* methods is ignored

[robertphillips, 2012-05-09 21:54:53]

This adds no new warnings on Windows but could prevent future errors

[reed1, 2012-05-10 12:03:51]

What future errors are we preventing? I supported this warning in a few places
where we were leaving a parameter in an uninitialized state (e.g.
SkMatrix::invert), as the function's result indicated that the method had
failed. Does that situation arise here?

[robertphillips, 2012-05-10 12:18:05]

I made the error of using MakeEmpty where I should've used setEmpty. This error
did leave the object I was invoking it on in an unexpected state (albeit in a
different manner than in the SkMatrix::invert case). It seems like there should
be no circumstances in which you should be calling the Make* methods w/o using
their return value.

[robertphillips, 2012-05-10 14:51:20]

Received verbal LGTM from Mike

committed as r3894.