Fix DropdownPopupWindow positioning on Android L.

Android L randomly changed the default vertical offset for ListPopupWindow to
-10dp by changing Widget.ListPopupWindow style to set dropDownVerticalOffset
to -10dp. This CL sets it explictly to the padding of our background image.

BUG=419444

Review URL: https://codereview.chromium.org/723843002

Cr-Commit-Position: refs/heads/master@{#303995}

ui/android/java/src/org/chromium/ui/DropdownPopupWindow.java

@@ -74,6 +74,9 @@ public void onDismiss() {
         });
 
         setAnchorView(mAnchorView);
+        Rect originalPadding = new Rect();
+        getBackground().getPadding(originalPadding);
+        setVerticalOffset(-originalPadding.top);
     }
 
     /**
@@ -111,8 +114,8 @@ public void show() {
         setInputMethodMode(INPUT_METHOD_NEEDED);
 
         int contentWidth = measureContentWidth();
-        float contentWidthInDip = contentWidth /
-                mContext.getResources().getDisplayMetrics().density;
+        float contentWidthInDip = contentWidth
+                / mContext.getResources().getDisplayMetrics().density;
         if (contentWidthInDip > mAnchorWidth) {
             setContentWidth(contentWidth);
             final Rect displayFrame = new Rect();