On desktop, this saves just over 5% of the time in the SkSL compiler.
As written, the code will now build either way, so it's much easier to
switch back (or even have some platforms use SkString, if that's ever
required).
Bug: skia:
Change-Id: I634f26a4f6fcb404e59bda6a5c6a21a9c6d73c0b
Reviewed-on: https://skia-review.googlesource.com/34381
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
