Bug 1344893 - Part 1: Report Navigation Timing into Telemetry. r=smaug, data-review=bsmedberg

We only need metric from top level content document, but we dont have
any information about it in nsDOMNavigationTiming, so I add a weak
reference which points to nsDocShell.

MozReview-Commit-ID: GiJigRLYHNV

docshell/base/nsDocShell.cpp

@@ -1788,7 +1788,7 @@ nsDocShell::MaybeInitTiming()
   }
 
   if (!mTiming) {
-    mTiming = new nsDOMNavigationTiming();
+    mTiming = new nsDOMNavigationTiming(this);
     canBeReset = true;
   }