Bug 1191961 - Ensure scrollframes are activated similarly in test and reference pages. r=kip

With APZ enabled, these tests activate scrollframes (by scrolling them) in the
test pages but not the reference pages. This causes differences in layerization
which causes reftests to fail by a few pixel differences in the scrollbars. By
making the scrollboxes will-change:scroll-position both the test and reference
pages have activated scrollframes, and so the layerization is the same in both.

--HG--
extra : commitid : BafhIev6bT

layout/reftests/scrolling/scroll-behavior-2.html

@@ -35,6 +35,7 @@
             width: 50px;
             height: 50px;
             overflow: scroll;
+            will-change: scroll-position;
         }
 
         #scroll_1, #scroll_2, #scroll_3, #scroll_6 {

layout/reftests/scrolling/scroll-behavior-3.html

@@ -35,6 +35,7 @@
             width: 50px;
             height: 50px;
             overflow: scroll;
+            will-change: scroll-position;
         }
 
         #scroll_1, #scroll_4, #scroll_6, #scroll_8 {

layout/reftests/scrolling/scroll-behavior-4.html

@@ -28,14 +28,15 @@
             top: 2000px;
             width: 20px;
             height: 20px;
-	    background: green;
-	    display: inline-block;
+            background: green;
+            display: inline-block;
         }
 
         .scroll_box {
             width: 50px;
             height: 50px;
             overflow: scroll;
+            will-change: scroll-position;
         }
 
         #scroll_1 {

layout/reftests/scrolling/scroll-behavior-6.html

@@ -35,6 +35,7 @@
             width: 50px;
             height: 50px;
             overflow: scroll;
+            will-change: scroll-position;
         }
 
     </style>