Fix the condition to disable the warning when building with Xcode

The warning -Wshorten-64-to-32 is automatically enabled by Xcode 5.1
when building for 64-bit architecture. Disable when the generator is
Xcode, not ninja.

BUG=359107

Review URL: https://codereview.chromium.org/231023002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262805 0039d316-1c4b-4281-b951-d872f2087c98

build/common.gypi

@@ -4380,7 +4380,7 @@
               ],
             }],
             ['OS=="ios" and target_subarch!="arm32" and \
-              "<(GENERATOR)"=="ninja"', {
+              "<(GENERATOR)"=="xcode"', {
               'OTHER_CFLAGS': [
                 # TODO(ios): when building Chrome for iOS on 64-bit platform
                 # with Xcode, the -Wshorted-64-to-32 warning is automatically