[scroll-animations] Allow null source in ScrollTimeline constructor

The spec was recently updated to differentiate between the source
being missing and the source being null. The former means that the
source should default to document.scrollingElement, and the latter
means that there is no source.

Note: In the CSS API, this translates to 'auto' and 'none',
respectively. Support for this has already landed in a previous CL.

Bug: 1100907
Change-Id: I1436b16d06b7a7ae0de55e2706a7b5d87c0290b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367837Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801085}

third_party/blink/renderer/core/animation/scroll_timeline.cc

@@ -95,7 +95,7 @@ Node* ResolveScrollSource(Element* scroll_source) {
 ScrollTimeline* ScrollTimeline::Create(Document& document,
                                        ScrollTimelineOptions* options,
                                        ExceptionState& exception_state) {
-  Element* scroll_source = options->scrollSource()
+  Element* scroll_source = options->hasScrollSource()
                                ? options->scrollSource()
                                : document.scrollingElement();
 

third_party/blink/renderer/core/animation/scroll_timeline_options.idl

@@ -14,7 +14,7 @@ enum ScrollDirection {
 enum ScrollTimelineAutoKeyword { "auto" };
 
 dictionary ScrollTimelineOptions {
-        Element? scrollSource = null;
+        Element? scrollSource;
         ScrollDirection orientation = "block";
         (DOMString or ScrollTimelineElementBasedOffset) startScrollOffset = "auto";
         (DOMString or ScrollTimelineElementBasedOffset)  endScrollOffset = "auto";

third_party/blink/web_tests/external/wpt/scroll-animations/constructor-expected.txt

 This is a testharness.js-based test.
-Found 60 tests; 58 PASS, 2 FAIL, 0 TIMEOUT, 0 NOTRUN.
+Found 61 tests; 59 PASS, 2 FAIL, 0 TIMEOUT, 0 NOTRUN.
 PASS A ScrollTimeline can be created with a scrollSource
 PASS A ScrollTimeline can be created with a non-scrolling scrollSource
-PASS A ScrollTimeline created with a null scrollSource should use the document.scrollingElement
+PASS A ScrollTimeline created with a null scrollSource should have no scrollSource
+PASS A ScrollTimeline created without a scrollSource should use the document.scrollingElement
 PASS A ScrollTimeline created with the default orientation should default to 'block'
 PASS 'block' is a valid orientation value
 PASS 'inline' is a valid orientation value

third_party/blink/web_tests/external/wpt/scroll-animations/constructor.html

@@ -49,13 +49,14 @@ test(t => {
 test(t => {
   assert_equals(
       new ScrollTimeline({scrollSource: null, timeRange: 100}).scrollSource,
-      document.scrollingElement);
-  // The default value for scrollSource should also map to the
-  // document.scrollingElement.
+      null);
+}, 'A ScrollTimeline created with a null scrollSource should have no scrollSource');
+
+test(t => {
   assert_equals(
       new ScrollTimeline({timeRange: 100}).scrollSource,
       document.scrollingElement);
-}, 'A ScrollTimeline created with a null scrollSource should use the document.scrollingElement');
+}, 'A ScrollTimeline created without a scrollSource should use the document.scrollingElement');
 
 // orientation