[Mac] Avoid "adding unknown subview" warning.

base/mac/sdk_forward_declarations.h

Index: base/mac/sdk_forward_declarations.h
diff --git a/base/mac/sdk_forward_declarations.h b/base/mac/sdk_forward_declarations.h
index c973eba1658a85006e331ba22da839e335e57361..5ed605da5a33703b7cb8a566de4bb7759b3173cd 100644
--- a/base/mac/sdk_forward_declarations.h
+++ b/base/mac/sdk_forward_declarations.h
@@ -235,6 +235,10 @@ BASE_EXPORT extern NSString* const NSAppearanceNameVibrantLight;
 @property(readonly) NSOperatingSystemVersion operatingSystemVersion;
 @end
 
+@interface NSLayoutConstraint (YosemiteSDK)
+@property(getter=isActive) BOOL active;
+@end
+
 @interface NSVisualEffectView (YosemiteSDK)
 - (void)setState:(NSVisualEffectState)state;
 @end
@@ -258,6 +262,22 @@ BASE_EXPORT extern NSString* const NSAppearanceNameVibrantLight;
 
 #endif  // MAC_OS_X_VERSION_10_10
 
+// Once Chrome no longer supports OSX 10.10, everything within this
+// preprocessor block can be removed.
+#if !defined(MAC_OS_X_VERSION_10_11) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_11
+
+@interface NSLayoutXAxisAnchor : NSObject

[yunchao, 2016/10/12 04:18:56]

Sorry to bother you, I failed to build chrome on MacOSX (both on Mac 10.11 and
10.12). The build error showed that there is duplicated definition for this
class. It has been defined in MacOSX system lib.

Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks/AppKit.framework/Headers/NSLayoutAnchor.h:50:12:
note: previous definition is here
@interface NSLayoutXAxisAnchor : NSLayoutAnchor<NSLayoutXAxisAnchor*>

Could you help to verify that Chrome can build on MacOSX 10.11/10.12? 

BTW, do we have MacOSX 10.11/10.12 buildbots before merge a patch? 

[tapted, 2016/10/12 04:22:34]

I think this lines needs to be

@interface NSLayoutXAxisAnchor (ElCapitanSDK) : NSObject

does that fix the error for you?
We run tests on the OSes, but don't iterate all developer build configurations -
Apple update them too regularly. Still, it's important to fix this, so if the
above works for you I'll land a quick fix.

[yunchao, 2016/10/12 04:30:37]

Still not work. But the error is "expected unqualified-id" now, the error at the
":"

+- (NSLayoutConstraint*)constraintEqualToAnchor:(id)anchor constant:(CGFloat)c;
+@end
+
+@interface NSView (ElCapitanSDK)
+@property(readonly, strong) NSLayoutXAxisAnchor* leftAnchor;
+@property(readonly, strong) NSLayoutXAxisAnchor* rightAnchor;
+@end
+
+#endif  // MAC_OS_X_VERSION_10_11
+
 // Once Chrome no longer supports OSX 10.11, everything within this
 // preprocessor block can be removed.
 #if !defined(MAC_OS_X_VERSION_10_12) || \