Bug 1146249 - Tiles on the newtab page don't wrap properly [r=adw]

Calculate the appropriate cell width for computing the grid width.

--HG--
extra : rebase_source : ab9b132cc741c74318761521de7a9fa435af7bdc

browser/base/content/newtab/grid.js

@@ -190,9 +190,9 @@ let gGrid = {
     // Save the cell's computed height/width including margin and border
     if (this._cellMargin === undefined) {
       let refCell = document.querySelector(".newtab-cell");
-      this._cellMargin = parseFloat(getComputedStyle(refCell).marginTop) +
+      this._cellMargin = parseFloat(getComputedStyle(refCell).marginTop);
+      this._cellHeight = refCell.offsetHeight + this._cellMargin +
         parseFloat(getComputedStyle(refCell).marginBottom);
-      this._cellHeight = refCell.offsetHeight + this._cellMargin;
       this._cellWidth = refCell.offsetWidth + this._cellMargin;
     }