Use default dimensions the first time we load a window, but saved dimensions after that.

src/browser/atom-window.coffee

@@ -49,6 +49,7 @@ class AtomWindow
     loadSettings.devMode ?= false
     loadSettings.safeMode ?= false
     loadSettings.atomHome = process.env.ATOM_HOME
+    loadSettings.firstLoad = true
 
     # Only send to the first non-spec window created
     if @constructor.includeShellLoadTime and not @isSpec