Expose fit-into default this is window but in some cases you want to

use another `fit-into` target. This is in general handy when you’re using
`dynamic-align=true` and the element where you want your
`paper-dropdown-menu` want to fit into  is _not_ `window`
This only will work if issue [108 of
paper-menu-button](PolymerElements/paper-menu-button#108)
is fixed, because `paper-dropdown-menu` is using `paper-menu-button`

paper-dropdown-menu-light.html

@@ -285,6 +285,7 @@
       no-animations="[[noAnimations]]"
       on-iron-select="_onIronSelect"
       on-iron-deselect="_onIronDeselect"
+      fit-into="[[fitInto]]"
       opened="{{opened}}"
       close-on-activate
       allow-outside-scroll="[[allowOutsideScroll]]">
@@ -424,6 +425,13 @@
             value: 'right'
           },
 
+          /**
+           * The element to fit `this` into.
+           */
+          fitInto: {
+            type: Object
+          },
+
           /**
            * The orientation against which to align the menu dropdown
            * vertically relative to the dropdown trigger.

paper-dropdown-menu.html

@@ -97,6 +97,7 @@
       on-iron-select="_onIronSelect"
       on-iron-deselect="_onIronDeselect"
       opened="{{opened}}"
+      fit-into="[[fitInto]]"
       close-on-activate
       allow-outside-scroll="[[allowOutsideScroll]]">
       <div class="dropdown-trigger">
@@ -258,6 +259,13 @@
             value: 'top'
           },
 
+          /**
+           * The element to fit `this` into.
+           */
+          fitInto: {
+            type: Object
+          },
+
           /**
            * If true, the `horizontalAlign` and `verticalAlign` properties will
            * be considered preferences instead of strict requirements when