hide trivial helpers on canvas

BUG=skia:

Change-Id: I42d4ca92897bde5bd8e0575a0104b12b83134cef
Reviewed-on: https://skia-review.googlesource.com/8852
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
37 files changed