Ensure that if FrameView::isScrollable() is false, the assoc WebLayer is, too.

LayoutTests/compositing/overflow/ignore-main-thread-scroll-reasons-when-main-frame-not-scrollable.html

Index: LayoutTests/compositing/overflow/ignore-main-thread-scroll-reasons-when-main-frame-not-scrollable.html
diff --git a/LayoutTests/compositing/overflow/ignore-main-thread-scroll-reasons-when-main-frame-not-scrollable.html b/LayoutTests/compositing/overflow/ignore-main-thread-scroll-reasons-when-main-frame-not-scrollable.html
index 8e85e587cbc31ad5d0625c9cd141d59695eb44aa..478570cbb2519279c7a5e78ef509468b5d3b392c 100644
--- a/LayoutTests/compositing/overflow/ignore-main-thread-scroll-reasons-when-main-frame-not-scrollable.html
+++ b/LayoutTests/compositing/overflow/ignore-main-thread-scroll-reasons-when-main-frame-not-scrollable.html
@@ -77,10 +77,7 @@ function shouldIgnore() {
     }
 }
 
-function shouldNotIgnore() {
-    document.body.style.height = "3000px";
-    document.body.offsetTop;
-    
+function checkThatWeHaveNotIgnored() {
     if (window.internals) {
         var reasons = internals.mainThreadScrollingReasons(document);
         if (reasons === '')
@@ -95,6 +92,18 @@ function shouldNotIgnore() {
     }
 }
 
+function shouldNotIgnore() {
+    document.body.style.height = "3000px";
+    document.body.offsetTop;
+
+    checkThatWeHaveNotIgnored(); 
+
+    document.body.style.overflow = "hidden";
+    document.body.offsetTop;
+
+    checkThatWeHaveNotIgnored(); 
+}
+
 function runTests() {
     shouldIgnore();
     shouldNotIgnore();