Reland "Box shadow should be recorded as main thread scrolling reasons for non-root layers."

third_party/WebKit/Source/web/tests/data/two_scrollable_area.html

Index: third_party/WebKit/Source/web/tests/data/two_scrollable_area.html
diff --git a/third_party/WebKit/Source/web/tests/data/two_scrollable_area.html b/third_party/WebKit/Source/web/tests/data/two_scrollable_area.html
index b7dba5ed5defbed704114f6c9dc323287964d2d4..9de72889d904c42e32ce38b6c46ee2b65232b4a3 100644
--- a/third_party/WebKit/Source/web/tests/data/two_scrollable_area.html
+++ b/third_party/WebKit/Source/web/tests/data/two_scrollable_area.html
@@ -27,6 +27,11 @@
   clip-path: circle(115px at 20px 20px);
 }
 
+.box-shadow {
+  box-shadow: 10px 10px 5px #888888;
+  will-change:transform; /*This reason is not recorded for root layer*/
+}
+
 .content {
   height: 500px;
 }