Add -ftrapv to debug builds.

gyp/common_conditions.gypi

 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
     'SK_SUPPORT_GPU=<(skia_gpu)',
     'SK_SUPPORT_OPENCL=<(skia_opencl)',
     'SK_FORCE_DISTANCEFIELD_FONTS=<(skia_force_distancefield_fonts)',
   ],
   'conditions' : [
@@ skipping 380 matching lines @@
         'defines': [
           'SK_SAMPLES_FOR_X',
           'SK_BUILD_FOR_UNIX',
         ],
         'configurations': {
           'Coverage': {
             'cflags': ['--coverage'],
             'ldflags': ['--coverage'],
           },
           'Debug': {
+            'cflags': ['-ftrapv'],  # Crash on signed integer over/underflow.
           },
           'Release': {
             'cflags': [
               '-O<(skia_release_optimization_level)',
             ],
             'defines': [ 'NDEBUG' ],
           },
         },
         'conditions' : [
           [ 'skia_shared_lib', {
@@ skipping 160 matching lines @@
           # Android Text Tuning
           'SK_GAMMA_EXPONENT=1.4',
           'SK_GAMMA_CONTRAST=0.0',
         ],
         # Android defines a fixed gamma exponent instead of using SRGB
         'defines!': [
           'SK_GAMMA_SRGB',
         ],
         'configurations': {
           'Debug': {
-            'cflags': ['-g']
+            'cflags': ['-g', '-ftrapv']
           },
           'Release': {
             'cflags': ['-O2'],
           },
         },
         'libraries': [
           '-llog',
         ],
         'cflags': [
           '-fuse-ld=gold',
@@ skipping 41 matching lines @@
     [ 'skia_is_bot', {
       'defines': [ 'SK_CRASH_HANDLER' ],
     }],
 
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }