[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
+- (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) || \