Everyone gets a namespace {}.

Everyone gets a namespace {}.

If we include Sk4px.h, SkPMFloat.h, or SkNx.h into files with different
SIMD flags, that could cause different definitions of the same method.

Normally that's moot, because all the code inlines, but in Debug it tends not
to.  So in Debug, the linker picks one definition for us.  That breaks _someone_.

Wrapping everything in a namespace {} keeps the definitions separate.

Tested locally, it fixes this bug.
BUG=skia:3861

This code is not yet enabled in Chrome, so shouldn't affect the roll.
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1154523004