[Mac] Avoid "adding unknown subview" warning.

base/mac/sdk_forward_declarations.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file contains forward declarations for items in later SDKs than the
 // default one with which Chromium is built (currently 10.10).
 // If you call any function from this header, be sure to check at runtime for
 // respondsToSelector: before calling these functions (else your code will crash
 // on older OS X versions that chrome still supports).
 
@@ skipping 217 matching lines @@
 @end
 
 @interface NSWindow (YosemiteSDK)
 - (void)setTitlebarAppearsTransparent:(BOOL)flag;
 @end
 
 @interface NSProcessInfo (YosemiteSDK)
 @property(readonly) NSOperatingSystemVersion operatingSystemVersion;
 @end
 
+@interface NSLayoutConstraint (YosemiteSDK)
+@property(getter=isActive) BOOL active;
+@end
+
 @interface NSVisualEffectView (YosemiteSDK)
 - (void)setState:(NSVisualEffectState)state;
 @end
 
 @class NSVisualEffectView;
 
 #endif  // MAC_OS_X_VERSION_10_10
 
 // Once Chrome no longer supports OSX 10.10.2, everything within this
 // preprocessor block can be removed.
 #if !defined(MAC_OS_X_VERSION_10_10_3) || \
     MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_10_3
 
 @interface NSEvent (YosemiteSDK)
 @property(readonly) NSInteger stage;
 @end
 
 @interface NSView (YosemiteSDK)
 - (void)setPressureConfiguration:(NSPressureConfiguration*)aConfiguration;
 @end
 
 #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) || \
     MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12
 
 @interface NSWindow (SierraSDK)
 @property(class) BOOL allowsAutomaticWindowTabbing;
 @end
 
 #endif  // MAC_OS_X_VERSION_10_12
 
 // ----------------------------------------------------------------------------
 // The symbol for kCWSSIDDidChangeNotification is available in the
 // CoreWLAN.framework for OSX versions 10.6 through 10.10. The symbol is not
 // declared in the OSX 10.9+ SDK, so when compiling against an OSX 10.9+ SDK,
 // declare the symbol.
 // ----------------------------------------------------------------------------
 BASE_EXPORT extern "C" NSString* const kCWSSIDDidChangeNotification;
 
 #endif  // BASE_MAC_SDK_FORWARD_DECLARATIONS_H_