Set a "metrics_client_id" crash key instead of "guid" on Mac OS X

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 9d0a7c6b67421a77597f001353e08a97d7b6154a..193dc25d780d18283843ebb2f2681ba6b2becece 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1270,6 +1270,11 @@ const char kDisableSystemFullscreenForTesting[] =
 // chrome/browser/mac/relauncher.h.
 const char kRelauncherProcess[]             = "relauncher";
 
+// This is how the metrics client ID is passed from the browser process to its
+// children. With Crashpad, the metrics client ID is distinct from the crash
+// client ID.
+const char kMetricsClientID[]               = "metrics-client-id";
+
 #endif
 
 // Use bubbles for content permissions requests instead of infobars.