Fix visual rect for background box painting in composited scrollers.

third_party/WebKit/LayoutTests/fast/scroll-behavior/overflow-scroll-with-local-background-and-text.html

+<script>
+if (window.testRunner)
+    testRunner.waitUntilDone();
+
+onload = function() {
+    // Double rAF to ensure content is painted before scroll.
+    requestAnimationFrame(function() {
+        requestAnimationFrame(scrollToBottom);
+    });
+}
+
+function scrollToBottom() {
+    document.getElementById('scroller').scrollTop = 500;
+    if (window.testRunner)
+        window.testRunner.notifyDone();
+}
+</script>
+<style>
+#scroller {
+    background: gray local;
+    border: 10px solid green;
+    overflow: scroll;
+    width: 200px;
+    height: 200px;
+    will-change: transform;
+}
+
+#spacer {
+    height: 500px;
+}
+</style>
+<!-- Test that the background is correctly repainted when the scrolled content
+    grows in size. -->
+<div id="scroller">
+    <div id="spacer"></div>
+    <p>Text content</p>
+</div>