Suppress some no-op GraphicsContext calls

The CL avoids commiting deferred canvas saves for a few easy to catch
no-op GC calls.

This reduces the number of SkPicture draw ops by ~3%, and shows minor
(<1%) rasterization time gains.

R=schenney@chromium.org,senorblanco@chromium.org

Review URL: https://codereview.chromium.org/136583009

git-svn-id: svn://svn.chromium.org/blink/trunk@165353 bbb929c8-8fbe-4397-9dbb-9b2b20218538