Subclasses of SkCanvas may need to override the behavior here - for
example, any proxy or deferred canvas may not know its own size and
need to delegate to another object.
We'll also work on reducing use of this function
(https://skbug.com/3569), but some of the current uses seem to be
semantically necessary.
R=reed@google.com
BUG=skia:3566
Review URL: https://codereview.chromium.org/1022423002
68260fa1e9