[ScrollTimeline] Force compositing scrollers to run tests on compositor correctly

This is similar to crrev.com/c/2211048. cc::ScrollTimeline is considered
as inactive if the scroller is not composited (not in cc::ScrollTree).

Bug: None
Change-Id: Id933a41ba40f8a736692653a934f963d52c011be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216711Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771955}

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

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

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

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

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

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

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

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

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

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

third_party/blink/web_tests/external/wpt/scroll-animations/updating-the-finished-state.html

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