Add skia_clang_build gyp define, add -Wstring-conversion flag

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 203 matching lines @@
               '-g',
             ],
             'defines': [ 'NDEBUG' ],
           },
         },
         'cflags': [
           '-Wall',
           '-Wextra',
           # suppressions below here were added for clang
           '-Wno-unused-parameter',
-          '-Wno-c++11-extensions'
+          '-Wno-c++11-extensions',
         ],
         'conditions' : [
           [ 'skia_shared_lib', {
             'cflags': [
               '-fPIC',
             ],
             'defines': [
               'SKIA_DLL',
               'SKIA_IMPLEMENTATION=1',
             ],
@@ skipping 40 matching lines @@
           }],
           [ 'skia_asan_build', {
             'cflags': [
               '-fsanitize=address',
               '-fno-omit-frame-pointer',
             ],
             'ldflags': [
               '-fsanitize=address',
             ],
           }],
+          [ 'skia_clang_build', {

[borenet, 2013/08/21 14:00:58]

It's kind of unfortunate that we have to place this flag behind a variable which
must be explicitly supplied, but I don't know of a better way.  I guess we could
shell out to see if "clang" is in "echo $CC", but that seems fragile.

+            'cflags': [
+              '-Wstring-conversion',

[djsollen, 2013/08/21 14:09:37]

This will break our bots if we check it in.  Do we have a plan to handle that?

[robertphillips, 2013/08/21 14:12:51]

Do you mean because all the !""'s outside of what Chromium normally sees will
become errors?

[borenet, 2013/08/21 14:16:01]

This doesn't change the behavior of our bots, since they don't yet provide the
"skia_clang_build" define.  I'll have to add that in a separate CL, which will
then cause the bots to fail if the warning hasn't been fixed.

+            ],
+          }],
         ],
       },
     ],
 
     [ 'skia_os == "mac"',
       {
         'variables': {
           'mac_sdk%': '<!(python <(DEPTH)/tools/find_mac_sdk.py 10.6)',
         },
         'defines': [
@@ skipping 182 matching lines @@
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }
 
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