Collect UMA statistics on which chrome://flags lead to chrome restart on ChromeOS.

chrome/browser/about_flags.h

Index: chrome/browser/about_flags.h
diff --git a/chrome/browser/about_flags.h b/chrome/browser/about_flags.h
index 55859db985106a932db1ab706f74d13f416c2512..926c1f373ec341744b782b19c6d1d3a1594fdd94 100644
--- a/chrome/browser/about_flags.h
+++ b/chrome/browser/about_flags.h
@@ -6,6 +6,7 @@
 #define CHROME_BROWSER_ABOUT_FLAGS_H_
 
 #include <map>
+#include <set>
 #include <string>
 
 #include "base/command_line.h"
@@ -117,9 +118,12 @@ void ConvertFlagsToSwitches(FlagsStorage* flags_storage,
 
 // Compares a set of switches of the two provided command line objects and
 // returns true if they are the same and false otherwise.
+// If |out_difference| is not NULL, it's filled with set_symmetric_difference
+// between sets.
 bool AreSwitchesIdenticalToCurrentCommandLine(
     const base::CommandLine& new_cmdline,
-    const base::CommandLine& active_cmdline);
+    const base::CommandLine& active_cmdline,
+    std::set<CommandLine::StringType>* out_difference);
 
 // Differentiate between generic flags available on a per session base and flags
 // that influence the whole machine and can be said by the admin only. This flag