Find device-dependent disk stats file, and skip disk stats if not available.

Change-Id: I03afb85e3357dd4c2cf5effd98b194c71d77c71d

BUG=12171
TEST=unit tested

Review URL: http://codereview.chromium.org/6541007
diff --git a/metrics/metrics_daemon.h b/metrics/metrics_daemon.h
index dd61322..5252518 100644
--- a/metrics/metrics_daemon.h
+++ b/metrics/metrics_daemon.h
@@ -30,7 +30,7 @@
 
   // Initializes.
   void Init(bool testing, MetricsLibraryInterface* metrics_lib,
-            const char* diskstats_path);
+            std::string diskstats_path);
 
   // Does all the work. If |run_as_daemon| is true, daemonizes by
   // forking.
@@ -304,7 +304,7 @@
   long int write_sectors_;
 
   DiskStatsState diskstats_state_;
-  const char* diskstats_path_;
+  std::string diskstats_path_;
 };
 
 #endif  // METRICS_DAEMON_H_