Enable fastbuild=1 on clang/mac

Before:
$ GYP_DEFINES="disable_nacl=1 fastbuild=1" CC=clang CXX=clang++ python build/gyp_chromium
$ ninja -C out/Release -t clean
$ time PATH=/path/to/goma:/path/to/chromium/src/third_party/llvm-build/Release+Asserts/bin:$PATH ninja -C out/Release -j 50 chrome
..
real    20m28.627s
user    9m20.091s
sys     2m43.130s

After:
$ GYP_DEFINES="disable_nacl=1 fastbuild=1" CC=clang CXX=clang++ python build/gyp_chromium
$ ninja -C out/Release -t clean
$ time PATH=/path/to/goma:/path/to/chromium/src/third_party/llvm-build/Release+Asserts/bin:$PATH ninja -C out/Release -j 50 chrome
...
real    8m55.293s
user    9m33.869s
sys     2m29.931s

R=thakis,shinyak,ilevy
BUG=none

Review URL: https://chromiumcodereview.appspot.com/14445005

git-svn-id: http://src.chromium.org/svn/trunk/src/build@196121 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1837,6 +1837,9 @@
         'defines': ['ENABLE_HIDPI=1'],
       }],
       ['fastbuild!=0', {
+        'xcode_settings': {
+          'GCC_GENERATE_DEBUGGING_SYMBOLS': 'NO',
+        },
         'conditions': [
           ['clang==1', {
             # Clang creates chubby debug information, which makes linking very
@@ -1882,11 +1885,6 @@
                   },
                 },
               }],
-              ['OS=="mac"', {
-                'xcode_settings': {
-                  'GCC_GENERATE_DEBUGGING_SYMBOLS': 'NO',
-                },
-              }],
               ['OS=="linux" and fastbuild==2', {
                 'variables': {
                   'debug_extra_cflags': '-g0',