add a way to get code coverage

gyp/common_conditions.gypi

 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
   ],
   'conditions' : [
     [ 'skia_gpu == 1',
       {
         'defines': [
@@ skipping 187 matching lines @@
     ],
 
 
     [ 'skia_os in ["linux", "freebsd", "openbsd", "solaris", "nacl", "chromeos"]',
       {
         'defines': [
           'SK_SAMPLES_FOR_X',
           'SK_BUILD_FOR_UNIX',
         ],
         'configurations': {
+          'Coverage': {
+            'cflags': ['-g --coverage'],

[borenet, 2013/09/12 17:22:58]

We already provide '-g' in both Debug and Release... why do we need it again
here?

[mtklein, 2013/09/12 17:34:00]

Yeah, I suppose debug symbols aren't necessary, but they don't really hurt
anything.  Is there some reason to leave them out?  I just added them for
symmetry with the other build targets.

[borenet, 2013/09/12 17:40:02]

Whoa - sorry, I totally missed that this was a different configuration.  This is
fine.

+            'ldflags': ['--coverage'],
+          },
           'Debug': {
             'cflags': ['-g']
           },
           'Release': {
             'cflags': [
               '-O<(skia_release_optimization_level)',
               '-g',
             ],
             'defines': [ 'NDEBUG' ],
           },
@@ skipping 264 matching lines @@
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }
 
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