mac: Use stabs instead of dwarf when fastbuild=1

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 007b908f50664bb0f718b0737fd5a669d6fbdf51..14e78d11b0d17b8227d7dc70c6b7c5df9f515c5a 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -857,6 +857,14 @@
               'debug_extra_cflags': '-g0',
             },
           }],
+          ['OS=="mac"', {
+            'xcode_settings': {
+              # Goma needs to ship all symbol information over the wire.
+              # <(debug_extra_cflags) will still generate -g1 STABS information,
+              # which is smaller than -gdwarf-2 information.
+              'GCC_GENERATE_DEBUGGING_SYMBOLS': 'NO',
+            },
+          }],
         ],  # conditions for fastbuild.
       }],  # fastbuild!=0
       ['selinux==1', {