Bug 1238137 - Telemetry pings for main thread autoscrolling. r=mconley

--HG--
extra : commitid : DbDCoiFw70B
extra : rebase_source : 4c1ffe4eca68b4da86da32d0467890b10a386f3c
extra : histedit_source : 0d8990d8229788a691ff14298529787c5de6a253

toolkit/content/browser-content.js

@@ -207,6 +207,9 @@ var ClickEventHandler = {
       this._scrollErrorX = (desiredScrollX - actualScrollX);
     }
 
+    const kAutoscroll = 15;  // defined in mozilla/layers/ScrollInputMethods.h
+    Services.telemetry.getHistogramById("SCROLL_INPUT_METHODS").add(kAutoscroll);
+
     if (this._scrollable instanceof content.Window) {
       this._scrollable.scrollBy(actualScrollX, actualScrollY);
     } else { // an element with overflow