Add skia_clang_build gyp define, add -Wstring-conversion flag

gyp/common_conditions.gypi

Index: gyp/common_conditions.gypi
===================================================================
--- gyp/common_conditions.gypi	(revision 10839)
+++ gyp/common_conditions.gypi	(working copy)
@@ -221,7 +221,7 @@
           '-Wextra',
           # suppressions below here were added for clang
           '-Wno-unused-parameter',
-          '-Wno-c++11-extensions'
+          '-Wno-c++11-extensions',
         ],
         'conditions' : [
           [ 'skia_shared_lib', {
@@ -282,6 +282,11 @@
               '-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.

+            ],
+          }],
         ],
       },
     ],