Update SkPMFloat API a bit.

bench/PMFloatBench.cpp

 #include "Benchmark.h"
 #include "SkPMFloat.h"
 
 // Used to prevent the compiler from optimizing away the whole loop.
 volatile uint32_t blackhole = 0;
 
 // Not a great random number generator, but it's very fast.
 // The code we're measuring is quite fast, so low overhead is essential.
 static uint32_t lcg_rand(uint32_t* seed) {
     *seed *= 1664525;
@@ skipping 12 matching lines @@
         uint32_t junk = 0;
         uint32_t seed = 0;
         for (int i = 0; i < loops; i++) {
         #ifdef SK_DEBUG
             // Our SkASSERTs will remind us that it's technically required that we premultiply.
             SkPMColor c = SkPreMultiplyColor(lcg_rand(&seed));
         #else
             // But it's a lot faster not to, and this code won't really mind the non-PM colors.
             SkPMColor c = lcg_rand(&seed);
         #endif
-            SkPMFloat pmf;
-            pmf.set(c);
+            SkPMFloat pmf = SkPMFloat::FromPMColor(c);
             SkPMColor back = fClamp ? pmf.clamped() : pmf.get();
             junk ^= back;
         }
         blackhole ^= junk;
     }
 
     bool fClamp;
 };
 DEF_BENCH(return new PMFloatBench( true);)
 DEF_BENCH(return new PMFloatBench(false);)