Update Element API for CSSOM smooth scrolling to match the spec

LayoutTests/fast/scroll-behavior/subframe-scrollLeft-expected.txt

Index: LayoutTests/fast/scroll-behavior/subframe-scrollLeft-expected.txt
diff --git a/LayoutTests/fast/scroll-behavior/subframe-scrollLeft-expected.txt b/LayoutTests/fast/scroll-behavior/subframe-scrollLeft-expected.txt
index 19beda79ba6ce590f2343c4ff30417b633c7abbd..d1e30f5c4cbbf9e3da3f230afe9473e02220df69 100644
--- a/LayoutTests/fast/scroll-behavior/subframe-scrollLeft-expected.txt
+++ b/LayoutTests/fast/scroll-behavior/subframe-scrollLeft-expected.txt
@@ -3,16 +3,10 @@ Test that setting scrollLeft on a subframe works with both scroll behaviors
 
 This is a testharness.js-based test.
 PASS Scroll x:1, y:0, smooth:false 
-PASS Scroll x:2, y:0, smooth:false 
-PASS Scroll x:3, y:0, smooth:false 
 PASS Scroll x:4, y:0, smooth:false 
-FAIL Scroll x:10, y:0, smooth:true assert_equals: expected "4, 0" but got "10, 0"
-FAIL Scroll x:20, y:0, smooth:true assert_equals: expected "10, 0" but got "20, 0"
-FAIL Scroll x:30, y:0, smooth:true assert_equals: expected "20, 0" but got "30, 0"
-FAIL Scroll x:40, y:0, smooth:true assert_equals: expected "30, 0" but got "40, 0"
-FAIL Scroll x:4000, y:0, smooth:true assert_equals: expected "40, 0" but got "4000, 0"
-FAIL Scroll x:15, y:0, smooth:true assert_equals: expected "4000, 0" but got "15, 0"
-FAIL Scroll x:4100, y:0, smooth:true assert_equals: expected "15, 0" but got "4100, 0"
-FAIL Scroll x:10, y:0, smooth:true assert_equals: expected "4100, 0" but got "10, 0"
+PASS Scroll x:20, y:0, smooth:true 
+PASS Scroll x:40, y:0, smooth:true 
+PASS Scroll x:4000, y:0, smooth:true 
+PASS Scroll x:10, y:0, smooth:true 
 Harness: the test ran to completion.