Switch to CodeMirror simplescrollbars addon

Instead of implementing the scrollbar inside of Gerrit, use the
simplescrollbars addon available with CodeMirror 4.9. The addon has
handlers for scroll wheel support and other basic browser behavior
that was broken or missing in Gerrit's own scrollbar replacement.

Markers in the overview bar can now be simpler widgets that track the
vertical scale ratio of the CM3 instance and position accordingly.

Change-Id: Ib117668ee0f85955d1e47034e32676bc9796d0d2

lib/codemirror/cm.defs

@@ -1,6 +1,7 @@
 CM_CSS = [
   'lib/codemirror.css',
   'addon/dialog/dialog.css',
+  'addon/scroll/simplescrollbars.css',
 ]
 
 CM_THEMES = [
@@ -16,6 +17,7 @@ CM_JS = [
   'lib/codemirror.js',
   'keymap/vim.js',
   'addon/dialog/dialog.js',
+  'addon/scroll/simplescrollbars.js',
   'addon/search/searchcursor.js',
   'addon/search/search.js',
   'addon/selection/mark-selection.js',