Add mixedtextblob gm and a simple bench

gyp/bench.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'include_dirs': [
     '../src/core',
     '../src/effects',
     '../src/gpu',
     '../src/utils',
@@ skipping 87 matching lines @@
     '../bench/RegionContainBench.cpp',
     '../bench/RepeatTileBench.cpp',
     '../bench/RotatedRectBench.cpp',
     '../bench/ScalarBench.cpp',
     '../bench/ShaderMaskBench.cpp',
     '../bench/SkipZeroesBench.cpp',
     '../bench/SortBench.cpp',
     '../bench/StrokeBench.cpp',
     '../bench/TableBench.cpp',
     '../bench/TextBench.cpp',
+    '../bench/TextBlobBench.cpp',
     '../bench/TileBench.cpp',
     '../bench/VertBench.cpp',
     '../bench/WritePixelsBench.cpp',
     '../bench/WriterBench.cpp',
     '../bench/XfermodeBench.cpp',
   ],
 }