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..420f86acf305f18693a122e83d9c812ce51ce119 100644
--- a/chrome/browser/about_flags.h
+++ b/chrome/browser/about_flags.h
@@ -6,10 +6,12 @@
 #define CHROME_BROWSER_ABOUT_FLAGS_H_
 
 #include <map>
+#include <set>
 #include <string>
 
 #include "base/command_line.h"
 #include "base/strings/string16.h"
+#include "chrome/browser/about_flags_switches_histogram_ids.h"
 
 class PrefService;
 
@@ -58,6 +60,12 @@ struct Experiment {
     const char* command_line_switch;
     // Simple switches that have no value should use "" for command_line_value.
     const char* command_line_value;
+
+    // ID of command_line_switch in UMA histograms.
+    // On ChromeOS experiments lead to chrome process restart when user logs in.
+    // IDs of switches that lead to restart are reported.
+    // Must be zero for entries without command_line_switches.
+    const int command_line_switch_histogram_id;
   };
 
   // The internal name of the experiment. This is never shown to the user.
@@ -85,11 +93,16 @@ struct Experiment {
   const char* command_line_switch;
   // Simple switches that have no value should use "" for command_line_value.
   const char* command_line_value;
+  // ID of command_line_switch in UMA histograms.
+  // (See Choice::command_line_switch_histogram_id.)
+  // Must be zero for entries without command_line_switches.
+  const int command_line_switch_histogram_id;
 
   // For ENABLE_DISABLE_VALUE, the command line switch and value to explictly
   // disable the feature.
   const char* disable_command_line_switch;
   const char* disable_command_line_value;
+  const int disable_command_line_switch_histogram_id;
 
   // This is used if type is MULTI_VALUE.
   const Choice* choices;
@@ -115,11 +128,18 @@ void ConvertFlagsToSwitches(FlagsStorage* flags_storage,
                             base::CommandLine* command_line,
                             SentinelsMode sentinels);
 
+// Returns a map <command_line_switch, switch_histogram_id>.
+// Map is created if it doesn't exist.
+const std::map<std::string, int>& GetSwitchesHistogramId();
+
 // 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