Cleanup: Pass std::string as const reference if possible

chrome/browser/chromeos/drive/file_system_util.h

Index: chrome/browser/chromeos/drive/file_system_util.h
diff --git a/chrome/browser/chromeos/drive/file_system_util.h b/chrome/browser/chromeos/drive/file_system_util.h
index 370c7524f087665feacc54e1fb6e81aea9528623..6dc6021f38b26767e66cfcf84b21064eaec6613d 100644
--- a/chrome/browser/chromeos/drive/file_system_util.h
+++ b/chrome/browser/chromeos/drive/file_system_util.h
@@ -31,7 +31,8 @@ base::FilePath GetDriveMountPointPath(Profile* profile);
 
 // Returns the Drive mount point path, which looks like
 // "/special/drive-<username_hash>", when provided with the |user_id_hash|.
-base::FilePath GetDriveMountPointPathForUserIdHash(std::string user_id_hash);
+base::FilePath GetDriveMountPointPathForUserIdHash(
+    const std::string& user_id_hash);
 
 // Returns true if the given path is under the Drive mount point.
 bool IsUnderDriveMountPoint(const base::FilePath& path);