Update SkPMFloat API a bit.

src/opts/SkPMFloat_none.h

 #include "SkColorPriv.h"
 
-inline void SkPMFloat::set(SkPMColor c) {
-    this->setA(SkGetPackedA32(c));
-    this->setR(SkGetPackedR32(c));
-    this->setG(SkGetPackedG32(c));
-    this->setB(SkGetPackedB32(c));
+inline SkPMFloat::SkPMFloat(SkPMColor c) {
+    *this = SkPMFloat::FromARGB(SkGetPackedA32(c),
+                                SkGetPackedR32(c),
+                                SkGetPackedG32(c),
+                                SkGetPackedB32(c));
     SkASSERT(this->isValid());
 }
 
 inline SkPMColor SkPMFloat::get() const {
     SkASSERT(this->isValid());
     return SkPackARGB32(this->a()+0.5f, this->r()+0.5f, this->g()+0.5f, this->b()+0.5f);
 }
 
 inline SkPMColor SkPMFloat::clamped() const {
     float a = this->a(),
           r = this->r(),
           g = this->g(),
           b = this->b();
     a = a < 0 ? 0 : (a > 255 ? 255 : a);
     r = r < 0 ? 0 : (r > 255 ? 255 : r);
     g = g < 0 ? 0 : (g > 255 ? 255 : g);
     b = b < 0 ? 0 : (b > 255 ? 255 : b);
     return SkPackARGB32(a+0.5f, r+0.5f, g+0.5f, b+0.5f);
 }