Generate a warning if the return value of FilePath::Append is ignored.

base/file_path.h

Index: base/file_path.h
diff --git a/base/file_path.h b/base/file_path.h
index 6bb426f7e1566e8f642926ecd7e578e637d689c8..1ebb299b76e9f08b06d22eef1a72829c067c7f6a 100644
--- a/base/file_path.h
+++ b/base/file_path.h
@@ -67,6 +67,7 @@
 
 #include <string>
 
+#include "base/compiler_specific.h"
 #include "base/basictypes.h"
 
 // Windows-style drive letter support and pathname separator characters can be
@@ -141,7 +142,7 @@ class FilePath {
   // If this object's path is kCurrentDirectory, a new FilePath corresponding
   // only to |component| is returned.  |component| must be a relative path;
   // it is an error to pass an absolute path.
-  FilePath Append(const StringType& component) const;
+  FilePath Append(const StringType& component) const WARN_UNUSED_RESULT;
 
   // Returns true if this FilePath contains an absolute path.  On Windows, an
   // absolute path begins with either a drive letter specification followed by