Make iossim binary work on Xcode 5 and 6.

The CL changes most compile-time checks to be runtime-checks so that
an iossim binary built agains Xcode 6 will still work against Xcode 5.

BUG=None

Review URL: https://codereview.chromium.org/494583004

Cr-Commit-Position: refs/heads/master@{#291681}

testing/iossim/iossim.gyp

@@ -25,6 +25,13 @@
               'defines': [
                 'IOSSIM_USE_XCODE_6',
               ],
+              'xcode_settings': {
+                # The CoreSimulator.h file generated by class-dump defines a
+                # property of type |NSString*| and a setter for the property
+                # that takes a parameter of type |id|. This type mismatch causes
+                # a compiler warning, so turn off -Werror.
+                'GCC_TREAT_WARNINGS_AS_ERRORS': 'NO',
+              },
               'actions': [
                 {
                   'action_name': 'generate_dvt_foundation_header',