moebius#223: Consider blocking top level window data: URIs (part 1/3 without tests)

https://github.com/MoonchildProductions/moebius/pull/223

netwerk/base/nsIOService.h

@@ -95,6 +95,8 @@ public:
 
     bool IsLinkUp();
 
+    static bool BlockToplevelDataUriNavigations();
+
     // Used to trigger a recheck of the captive portal status
     nsresult RecheckCaptivePortal();
 private:
@@ -176,6 +178,8 @@ private:
 
     static bool                          sTelemetryEnabled;
 
+    static bool                          sBlockToplevelDataUriNavigations;
+
     // These timestamps are needed for collecting telemetry on PR_Connect,
     // PR_ConnectContinue and PR_Close blocking time.  If we spend very long
     // time in any of these functions we want to know if and what network