This is a follow up to https://codereview.chromium.org/338913002/. More
tweaks to allow migrating clients to the new API.
1) switch the shim call direction willSave(SaveFlags) -> willSave()
(internal users are still using the former, so external overriders of
the latter will not be notified otherwise - doh)
2) ensure willSave() stays visible in SkProxyCanvas (Chromium's
TimingCanvas attempts to call it explicitly).
BUG=skia:2297
R=reed@google.com
Author: fmalita@chromium.org
Review URL: https://codereview.chromium.org/334393008
6ca763f362