Adding new benchmark to test image decoding performance.

gyp/iOSShell.gyp

 #
 {
   'conditions' : [
     [ 'skia_os == "ios"', {
       'targets': [
         {
           'target_name': 'iOSShell',
           'type': 'executable',
           'mac_bundle' : 1,
           'includes': [
             'bench.gypi',
             'dm.gypi',
           ],
           'dependencies': [
             'tools.gyp:crash_handler',
             'tools.gyp:timer',
             'views.gyp:views',
             'xml.gyp:xml',
           ],
           'sources': [
+            '../bench/DecodingBench.cpp',
+            '../bench/DecodingSubsetBench.cpp',
             '../bench/GMBench.cpp',
             '../bench/RecordingBench.cpp',
             '../bench/SKPBench.cpp',
             '../bench/nanobench.cpp',
             '../tests/skia_test.cpp',
             '../tools/iOSShell.cpp',
             '../src/views/mac/SkEventNotifier.mm',
             '../experimental/iOSSampleApp/SkiOSSampleApp-Base.xcconfig',
             '../experimental/iOSSampleApp/SkiOSSampleApp-Debug.xcconfig',
             '../experimental/iOSSampleApp/SkiOSSampleApp-Release.xcconfig',
@@ skipping 50 matching lines @@
               'dependencies': [
                 'gputest.gyp:skgputest',
               ],
             }],
           ],
         },
       ],
     }],
   ]
 }