Start collecting two new WebSocket histograms.

Source/modules/websockets/WebSocketHandshake.h

Index: Source/modules/websockets/WebSocketHandshake.h
diff --git a/Source/modules/websockets/WebSocketHandshake.h b/Source/modules/websockets/WebSocketHandshake.h
index 803f70883f496224f0b9f2dbd249383746a04cfb..59852a0b94dd9bbb76bb8f0d7fd6526e29368d4b 100644
--- a/Source/modules/websockets/WebSocketHandshake.h
+++ b/Source/modules/websockets/WebSocketHandshake.h
@@ -46,8 +46,11 @@ class ScriptExecutionContext;
 class WebSocketHandshake {
     WTF_MAKE_NONCOPYABLE(WebSocketHandshake); WTF_MAKE_FAST_ALLOCATED;
 public:
+    // This enum is reused for histogram. When this needs to be modified, add a
+    // new enum for histogram and convert mode values into values in the new
+    // enum to keep new data consistent with old one.
     enum Mode {
-        Incomplete, Normal, Failed, Connected
+        Incomplete, Normal, Failed, Connected, ModeMax
     };
     WebSocketHandshake(const KURL&, const String& protocol, ScriptExecutionContext*);
     ~WebSocketHandshake();