Add skia_tsan_build to match skia_asan_build and fix one example race.

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 268 matching lines @@
           }],
           [ 'skia_asan_build', {
             'cflags': [
               '-fsanitize=address',
               '-fno-omit-frame-pointer',
             ],
             'ldflags': [
               '-fsanitize=address',
             ],
           }],
+          [ 'skia_tsan_build', {
+            'cflags': [
+              '-fsanitize=thread',
+              '-fno-omit-frame-pointer',
+            ],
+            'ldflags': [
+              '-fsanitize=thread',
+            ],
+          }],
           [ 'skia_clang_build', {
             'cflags': [
               '-Wstring-conversion',
             ],
           }],
         ],
       },
     ],
 
     [ 'skia_os == "mac"',
@@ skipping 187 matching lines @@
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }
 
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