have start_pipeline() return limit again

This is spooky.
I don't quite yet understand why, but this makes things much faster.

Performance regressed across the board when we no longer needed the
value and changed it to return void:

    https://perf.skia.org/e/?begin=1496176469&keys=6994&xbaroffset=28513

You can see similar regressions following this Chromium bug link.
BUG=chromium:729237

Change-Id: I68371b0456014f909acf819aca52aa4f4f187460
Reviewed-on: https://skia-review.googlesource.com/18580
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
7 files changed