SkPMFloat: fewer internal this->isValid() assertions.

  Each of these conversion functions now only asserts is output is valid.
  For SkPMColor -> SkPMFloat, we assert isValid().
  For SkPMFloat -> SkPMColor, we SkPMColorAssert.

 #floats

BUG=skia:
BUG=skia:3592

Review URL: https://codereview.chromium.org/1055093002
5 files changed