Move SkiaCanvas::Clip above SkiaCanvas::SkiaCanvas

After upgrading libc++, there are errors complaining about the the
incomplete Clip type in the "std::vector<Clip> mClipStack" member of
SkiaCanvas. The "SkiaCanvas::SkiaCanvas() {}" ctor can call ~vector
(maybe when an exception is thrown), and ~vector needs to know the size
of Clip.

(I'm not sure why this wasn't a problem before, but this seems to fix
the problem.)

Bug: http://b/175635923
Test: treehugger
Change-Id: I1d689c91be3b0530272836eeb727709ff1084545