[ninja/mac] Add xcode_emulation support for -Wobjc-missing-property-synthesis.
[ninja/mac] Add xcode_emulation support for -Wobjc-missing-property-synthesis.

CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS maps to
-Wobjc-missing-property-synthesis

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/32193007
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp