Adding new benchmark to test image decoding performance.

gyp/bench.gyp

 # GYP file to build performance testbench.
 #
 {
   'includes': [
     'apptype_console.gypi',
   ],
   'targets': [
     {
       'target_name': 'nanobench',
       'type': 'executable',
       'sources': [
         '../gm/gm.cpp',
+        '../bench/DecodingBench.cpp',
+        '../bench/DecodingSubsetBench.cpp',
         '../bench/GMBench.cpp',
         '../bench/RecordingBench.cpp',
         '../bench/SKPBench.cpp',
         '../bench/nanobench.cpp',
       ],
       'includes': [
         'bench.gypi',
         'gmslides.gypi',
       ],
       'dependencies': [
         'flags.gyp:flags_common',
         'jsoncpp.gyp:jsoncpp',
         'skia_lib.gyp:skia_lib',
         'tools.gyp:crash_handler',
         'tools.gyp:proc_stats',
         'tools.gyp:timer',
       ],
       'conditions': [
         ['skia_android_framework', {
           'libraries': [
             '-lskia',
           ],
         }],
       ],
     },
   ],
 }