Add -Wno-invalid-offsetof to Mac and iOS builds too.

We use this on Linux already, but for whatever reason wasn't a problem on iOS
until using it in SkPaint.  Mac 10.7 and 10.8 are showing this warning too,
but seems -Werror is not enabled.

CQ_EXTRA_TRYBOTS=tryserver.skia:Build-Mac10.7-Clang-Arm7-Release-iOS-Trybot

BUG=skia:

Review URL: https://codereview.chromium.org/637593002

gyp/common_conditions.gypi

@@ -509,6 +509,7 @@
           'WARNING_CFLAGS': [
             '-Wall',
             '-Wextra',
+            '-Wno-invalid-offsetof',
             '-Wno-unused-parameter',
             '-Wno-uninitialized',  # Disabled because we think GCC 4.2 is bad at this.
           ],
@@ -551,6 +552,7 @@
           'SDKROOT': 'iphoneos',
           'TARGETED_DEVICE_FAMILY': '1,2',
           'OTHER_CPLUSPLUSFLAGS': [
+            '-Wno-invalid-offsetof',
             '-fvisibility=hidden',
             '-fvisibility-inlines-hidden',
           ],