experimental tight-bounds

experimental tight-bounds

not sure about api -- perhaps it could just return the bounds, and make them 0,0,0,0 if the path
is empty -- the caller can trivially know if the path is empty themselves.

BUG=skia:

Change-Id: I2dbb861e8d981b27c5a6833643977f5bd6802217
Reviewed-on: https://skia-review.googlesource.com/7989
Reviewed-by: Cary Clark <caryclark@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>