Add benchmark for PremultiplyAlpha and UnpremultiplyAlpha in Skia

gyp/bench.gypi

 # sources and conditions used in skia's bench.gyp and chromium's skia.gyp
 #
 {
   'sources': [
     '../bench/benchmain.cpp',
     '../bench/SkBenchmark.h',
     '../bench/SkBenchmark.cpp',
 
     '../bench/AAClipBench.cpp',
     '../bench/BicubicBench.cpp',
@@ skipping 28 matching lines @@
     '../bench/MemsetBench.cpp',
     '../bench/MergeBench.cpp',
     '../bench/MorphologyBench.cpp',
     '../bench/MutexBench.cpp',
     '../bench/PathBench.cpp',
     '../bench/PathIterBench.cpp',
     '../bench/PerlinNoiseBench.cpp',
     '../bench/PicturePlaybackBench.cpp',
     '../bench/PictureRecordBench.cpp',
     '../bench/ReadPixBench.cpp',
+    '../bench/PremulAndUnpremulAlphaOpsBench.cpp',
     '../bench/RectBench.cpp',
     '../bench/RectoriBench.cpp',
     '../bench/RefCntBench.cpp',
     '../bench/RegionBench.cpp',
     '../bench/RegionContainBench.cpp',
     '../bench/RepeatTileBench.cpp',
     '../bench/RTreeBench.cpp',
     '../bench/ScalarBench.cpp',
     '../bench/ShaderMaskBench.cpp',
     '../bench/SortBench.cpp',
@@ skipping 10 matching lines @@
     '../bench/TimerData.h',
     '../bench/TimerData.cpp',
   ],
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: