UMA: Set Hardware model class on Mac.

base/sys_info.h

Index: base/sys_info.h
diff --git a/base/sys_info.h b/base/sys_info.h
index d24acdf6b434eddf991c7787dc4f4e10989e9003..660343d06f76831b2f6dceff35cce4bb40e212c0 100644
--- a/base/sys_info.h
+++ b/base/sys_info.h
@@ -51,6 +51,12 @@ class BASE_EXPORT SysInfo {
   // Returns system uptime in milliseconds.
   static int64 Uptime();
 
+  // Returns a descriptive string for the current machine model or an empty
+  // string if machime model is unknown or an error occured.
+  // e.g. MacPro1,1 on Mac.
+  // Only implemented on OS X, will return an empty string on other platforms.
+  static std::string HardwareModelName();
+
   // Returns the name of the host operating system.
   static std::string OperatingSystemName();