Add the -Wobjc-missing-property-synthesis flag.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 96e2fb79a55a8ab76b508638e2dccd4228544c8e..1e89ccdb015d2cf6a77b0e429e6737b694fab412 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -3069,6 +3069,8 @@
             '-Wall',
             '-Wendif-labels',
             '-Wextra',
+            # Warn if automatic synthesis is triggered.
+            '-Wobjc-missing-property-synthesis',

[Nico, 2012/11/14 21:06:55]

If you say 'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES' instead, then the
UI will know about this warning too. (ninja won't, but that'd be easy to add.)

If you want to keep the flag, it should probably go in the clang==1 block below

[Mark Mentovai, 2012/11/15 00:06:52]

There’s a symbolic name for this

CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS

which you can set to YES above (and then put a comment saying
-Wobjc-missing-property-synthesis per the rest of the compiler options). Does
that work?

We would also need to add ninja support for this symbolic name. We generally use
those symbolic names when they’re available.

             # Don't warn about unused function parameters.
             '-Wno-unused-parameter',
             # Don't warn about the "struct foo f = {0};" initialization
@@ -3315,7 +3317,7 @@
           # also seems to mean using the custom build of clang.
 
           # Don't use -Wc++0x-extensions, which Xcode 4 enables by default
-          # when buliding with clang. This warning is triggered when the
+          # when building with clang. This warning is triggered when the
           # override keyword is used via the OVERRIDE macro from
           # base/compiler_specific.h.
           'CLANG_WARN_CXX0X_EXTENSIONS': 'NO',
@@ -3328,7 +3330,7 @@
             '-Wno-unused-function',
             # See comments on this flag higher up in this file.
             '-Wno-unnamed-type-template-args',
-            # This (rightyfully) complains about 'override', which we use
+            # This (rightfully) complains about 'override', which we use
             # heavily.
             '-Wno-c++11-extensions',
           ],