Test sk_linear_to_srgb() less exhaustively in Debug mode.

tests/SRGBTest.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSRGB.h"
 #include "SkTypes.h"
 #include "Test.h"
@@ skipping 15 matching lines @@
     // Should be monotonic between 0 and 1.
     uint8_t prev = 0;
     for (float f = FLT_MIN; f <= 1.0f; ) {  // We don't bother checking denorm values.
         uint8_t srgb = linear_to_srgb(f);
 
         REPORTER_ASSERT(r, srgb >= prev);
         prev = srgb;
 
         union { float flt; uint32_t bits; } pun = { f };
         pun.bits++;
+        SkDEBUGCODE(pun.bits += 127);
         f = pun.flt;
     }
 }