Sync canvas proxy CTM (b/21945972)

SkiaCanvasProxy was being created with an identity transform, ignoring
any transform that may have been applied in Java (or C++) to the
android graphics Canvas it was proxy for. This CL makes sure the
DisplayListCanvas transform is propagated to the Proxy every time
asSkCanvas() is called.

We could instead move the code to the SkiaCanvasProxy constructor
if we got rid of the cached proxy on DisplayListCanvas; nobody's
using the proxy heavily enough that that should be a performance
hit at this time.

BUG:21945972
R=djsollen@google.com

Change-Id: I99ed1563802a2449bb9939cb67976cd60dd8611c