[ScrollTimeline] Run tests on cc if threaded compositing is enabled

The scroller in the updated tests is not composited by default. When we
run the tests with the virtual/threaded test suite, the tests can not
properly run on the compositor because cc::ScrollTimeline is considered
as inactive, i.e. scroller is not in cc::ScrollTree because it's not
composited.

This patch forces compositing the scrollers if the animation needs to
play.

Note that once all ScrollNodes including uncomposited ones are in the
compositor after the scroll unification (crbug.com/476553), we no longer
need to add will-change to the scrollers.

Bug: None
Change-Id: I0e379abb071c9bffc58f2167f4cac9f92edbb9da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2211048Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771161}

third_party/blink/web_tests/external/wpt/scroll-animations/element-based-offset-clamp.html

@@ -11,6 +11,7 @@
   overflow: scroll;
   height: 500px;
   width: 500px;
+  will-change: transform;
 }
 
 /* Disable scrollbars to simplify the calculations in the test. */

third_party/blink/web_tests/external/wpt/scroll-animations/element-based-offset.html

@@ -11,6 +11,7 @@
     overflow: auto;
     height: 500px;
     width: 500px;
+    will-change: transform;
   }
 
   .contents {

third_party/blink/web_tests/external/wpt/scroll-animations/finish-animation.html

@@ -12,6 +12,7 @@
   overflow: auto;
   height: 200px;
   width: 100px;
+  will-change: transform;
 }
 
 .contents {

third_party/blink/web_tests/external/wpt/scroll-animations/scroll-animation-inactive-timeline.html

@@ -10,6 +10,7 @@
     overflow: auto;
     height: 100px;
     width: 100px;
+    will-change: transform;
   }
   .contents {
     height: 1000px;

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

@@ -10,6 +10,7 @@
     overflow: auto;
     height: 100px;
     width: 100px;
+    will-change: transform;
   }
   .contents {
     height: 1000px;

third_party/blink/web_tests/external/wpt/scroll-animations/scroll-timeline-invalidation.html

@@ -11,6 +11,7 @@
     overflow: auto;
     height: 100px;
     width: 100px;
+    will-change: transform;
   }
   .contents {
     height: 1000px;

third_party/blink/web_tests/external/wpt/scroll-animations/setting-current-time.html

@@ -11,6 +11,7 @@
   overflow: auto;
   height: 200px;
   width: 100px;
+  will-change: transform;
 }
 .contents {
   height: 1000px;

third_party/blink/web_tests/external/wpt/scroll-animations/setting-playback-rate.html

@@ -11,6 +11,7 @@
   overflow: auto;
   height: 100px;
   width: 100px;
+  will-change: transform;
 }
 .contents {
   height: 1000px;

third_party/blink/web_tests/external/wpt/scroll-animations/setting-start-time.html

@@ -11,6 +11,7 @@
   overflow: auto;
   height: 200px;
   width: 100px;
+  will-change: transform;
 }
 .contents {
   height: 1000px;