hide accessBitmap from g3

BUG=skia:

Change-Id: I00e8121751e640ee3b449d27b6ccfcc1491964a7
Reviewed-on: https://skia-review.googlesource.com/5224
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed