Prevent a crash in CompositorAnimations::CheckCanStartEffectOnCompositor

The reason for the crash and the reason for the flaky layout tests are
the same, we didn't check the |layout_object| in that function.

Locally when I run the flaky layout tests, I can get at least 1 crash
every 10 runs. With this CL, I got no crash with 20 runs for each test.

Bug: 1018027, 1018596
Change-Id: Ib9113324d42950f280ef0125b34e453db1427282
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1883621Reviewed-by: Yi Gu <yigu@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711246}

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

@@ -269,7 +269,8 @@ CompositorAnimations::CheckCanStartEffectOnCompositor(
             DCHECK(keyframe_value->IsDouble() || keyframe_value->IsColor());
             // If a custom property is not used by CSS Paint, then we should not
             // support that on the compositor thread.
-            if (!layout_object->Style()->HasCSSPaintImagesUsingCustomProperty(
+            if (layout_object && layout_object->Style() &&
+                !layout_object->Style()->HasCSSPaintImagesUsingCustomProperty(
                     property.CustomPropertyName(),
                     layout_object->GetDocument()))
               reasons |= kUnsupportedCSSProperty;

third_party/blink/web_tests/TestExpectations

@@ -5738,10 +5738,6 @@ crbug.com/1017626 [ Linux ] external/wpt/compat/idlharness.window.html [ Pass Ti
 crbug.com/925325 [ Win ] storage/indexeddb/index-population.html [ Pass Timeout ]
 
 # Sheriff 2019-10-25
-crbug.com/1018027 virtual/threaded-no-composited-antialiasing/animations/custom-properties/registered-var-to-unregistered.html [ Pass Crash ]
-crbug.com/1018027 virtual/threaded-no-composited-antialiasing/animations/custom-properties/registered-var-fallback.html [ Pass Crash ]
-crbug.com/1018027 virtual/threaded-no-composited-antialiasing/animations/custom-properties/registered-var-to-registered-animating.html [ Pass Crash ]
-crbug.com/1018027 virtual/threaded-no-composited-antialiasing/animations/custom-properties/registered-var-to-unregistered-animating.html [ Pass Crash ]
 crbug.com/1018064 [ Mac10.13 ] virtual/threaded/http/tests/devtools/tracing/timeline-js/timeline-js-line-level-profile-end-to-end.js [ Pass Timeout ]
 
 # Sheriff 2019-10-30