Removed scroll start and end offset properties of ScrollTimeline

Made modifications spec'ed in
w3c/csswg-drafts#5803:
- Removed startOffset and endOffset attributes.
- Updated procedures based on spec changes, including allowing single
scroll offset of 'auto'.

Bug: 1094014
Change-Id: Id08c9ce1818bd561ac291ca3e1f99c22dbb15674

scroll-animations/constructor.html

@@ -138,10 +138,10 @@
 
 test(t => {
   let constructorFunc = function() {
-    new ScrollTimeline({timeRange: 100, scrollOffsets: ['auto']})
+    new ScrollTimeline({timeRange: 100, scrollOffsets: [CSS.percent(20), 'auto', CSS.percent(50)]})
   };
   assert_throws_js(TypeError, constructorFunc);
-}, 'Creating a ScrollTimeline with an scrollOffsets value of [\'auto\'] should throw');
+}, 'Creating a ScrollTimeline with an scrollOffsets value of [CSS.percent(20), \'auto\', CSS.percent(50)] should throw');
 
 const gValidScrollOffsetValues = [
   CSS.px(0),

scroll-animations/css/at-scroll-timeline-ignored.tentative.html

@@ -103,8 +103,7 @@
     let timeline4 = new ScrollTimeline({
       scrollSource: scroller4,
       timeRange: 1e13,
-      startScrollOffset: CSS.px(0),
-      endScrollOffset: CSS.px(100)
+      scrollOffsets: [CSS.px(0), CSS.px(100)]
     });
 
     element.getAnimations()[0].timeline = timeline4;

scroll-animations/multiple-scroll-offsets.html

@@ -118,7 +118,7 @@
     scrollSource: scroller,
     timeRange: scrollerSize,
     orientation: 'block',
-    scrollOffsets: [CSS.px(0), CSS.px(400), CSS.px(200)],
+    scrollOffsets: ['auto', CSS.px(400), CSS.px(200)],
   });
 
   scroller.scrollTop = 100;