Move wpt/css/composite-bgcolor-animation to wpt/css/css-backgrounds

The wpt/css/composite-bgcolor-animation was newly added in this CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2464428,
which is specifically to tests the ongoing work of composite background
color animation.

It turns out that we already have wpt/css/css-backgrounds that has
a lot of tests in it to test painting background color. So this CL
moves the tests under composite-bgcolor-animation to css-backgrounds,
and then make the entire css-backgrounds a virtual test suite. This
will ensure more test coverage.

As a result of creating a new virtual test suite, there are 4 tests
currently failing. IMHO that should not block this CL because this
feature is behind a flag.

Bug: 1148305
Change-Id: I3759d0f431d8bcdb5e73f6aec42462ccdb343101
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533586Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826921}

third_party/blink/web_tests/TestExpectations

@@ -3316,6 +3316,12 @@ crbug.com/435547 http/tests/cachestorage/serviceworker/ignore-search-with-creden
 crbug.com/697971 [ Mac10.12 ] virtual/text-antialias/selection/flexbox-selection-nested.html [ Skip ]
 crbug.com/697971 [ Mac10.12 ] virtual/text-antialias/selection/flexbox-selection.html [ Skip ]
 
+# [composite-bgcolor-animation]
+crbug.com/1148369 virtual/composite-bgcolor-animation/external/wpt/css/css-backgrounds/border-bottom-left-radius-010.xht [ Failure ]
+crbug.com/1148369 virtual/composite-bgcolor-animation/external/wpt/css/css-backgrounds/border-bottom-right-radius-010.xht [ Failure ]
+crbug.com/1148369 virtual/composite-bgcolor-animation/external/wpt/css/css-backgrounds/border-top-left-radius-010.xht [ Failure ]
+crbug.com/1148369 virtual/composite-bgcolor-animation/external/wpt/css/css-backgrounds/border-top-right-radius-010.xht [ Failure ]
+
 # [css-grid]
 crbug.com/921722 external/wpt/css/css-grid/abspos/descendant-static-position-001.html [ Failure ]
 crbug.com/921722 external/wpt/css/css-grid/abspos/descendant-static-position-002.html [ Failure ]

third_party/blink/web_tests/VirtualTestSuites

@@ -185,8 +185,9 @@
   },
   {
     "prefix": "composite-bgcolor-animation",
-    "bases": ["external/wpt/css/composite-bgcolor-animation"],
-    "args": ["--enable-blink-features=CompositeBGColorAnimation"]
+    "bases": ["external/wpt/css/css-backgrounds"],
+    "args": ["--enable-threaded-compositing",
+             "--enable-blink-features=CompositeBGColorAnimation"]
   },
   {
     "prefix": "composite-after-paint",