Convert SysInfo::AmountOfFreeDiskSpace from wstring to FilePath.

base/sys_info.h

Index: base/sys_info.h
diff --git a/base/sys_info.h b/base/sys_info.h
index 5469da545111832fa3cea84d6cf7c158220d02f9..7460fe09a66319242d56bc8a6b328af591f22c99 100644
--- a/base/sys_info.h
+++ b/base/sys_info.h
@@ -9,6 +9,8 @@
 
 #include <string>
 
+class FilePath;
+
 namespace base {
 
 class SysInfo {
@@ -26,7 +28,7 @@ class SysInfo {
 
   // Return the available disk space in bytes on the volume containing |path|,
   // or -1 on failure.
-  static int64 AmountOfFreeDiskSpace(const std::wstring& path);
+  static int64 AmountOfFreeDiskSpace(const FilePath& path);
 
   // Return true if the given environment variable is defined.
   // TODO: find a better place for HasEnvVar.