Add Xfermode bench. Also clear before rendering in bench (rather than after).

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 32 matching lines @@
     '../bench/RTreeBench.cpp',
     '../bench/ScalarBench.cpp',
     '../bench/ShaderMaskBench.cpp',
     '../bench/SortBench.cpp',
     '../bench/StrokeBench.cpp',
     '../bench/TableBench.cpp',
     '../bench/TextBench.cpp',
     '../bench/TileBench.cpp',
     '../bench/VertBench.cpp',
     '../bench/WriterBench.cpp',
+    '../bench/XfermodeBench.cpp',
 
     '../bench/SkBenchLogger.h',
     '../bench/SkBenchLogger.cpp',
     '../bench/TimerData.h',
     '../bench/TimerData.cpp',
   ],
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: