Add neutral as a SecurityState in the Security domain.

In light of marking passive mixed content as neutral, "neutral" is by far a better name.

BUG=522274

Review URL: https://codereview.chromium.org/1301823003

git-svn-id: svn://svn.chromium.org/blink/trunk@200787 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/devtools/protocol.json

@@ -1076,7 +1076,7 @@
             {
                 "id": "SecurityState",
                 "type": "string",
-                "enum": ["unknown", "http", "insecure", "warning", "secure"],
+                "enum": ["unknown", "http", "neutral", "insecure", "warning", "secure"],
                 "description": "The security level of a page or resource."
             },
             {