Mark http/tests/devtools/layers/layer-compositing-reasons.js slow

Just like other http/tests/devtools tests.

TBR=wangxianzhu@chromium.org

Bug: 859169
Change-Id: Ia5e2f45ebeb97c8f7946f47ba366277d5f1ed30f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040120Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738850}

third_party/blink/web_tests/SlowTests

@@ -92,7 +92,6 @@ crbug.com/902685 virtual/not-split-http-cache-not-site-per-process/http/tests/de
 crbug.com/902685 http/tests/devtools/wasm-isolated-code-cache/wasm-cache-test.js [ Slow ]
 crbug.com/902685 virtual/wasm-site-isolated-code-cache/http/tests/devtools/wasm-isolated-code-cache/wasm-cache-test.js [ Slow ]
 crbug.com/902685 virtual/split-http-cache-not-site-per-process/http/tests/devtools/isolated-code-cache/* [ Slow ]
-
 # Misc DevTools tests that are slow
 crbug.com/246190 [ Release ] http/tests/devtools/indexeddb/* [ Slow ]
 crbug.com/451577 [ Mac ] http/tests/devtools/extensions/extensions-sidebar.js [ Slow ]
@@ -114,6 +113,7 @@ crbug.com/451577 [ Mac ] http/tests/devtools/extensions/extensions-resources.js
 crbug.com/451577 [ Win10 ] http/tests/devtools/extensions/extensions-sidebar.js [ Slow ]
 crbug.com/451577 [ Mac ] http/tests/devtools/layers/layer-canvas-log.js [ Slow ]
 crbug.com/451577 [ Mac ] http/tests/devtools/network/network-domain-filter.js [ Slow ]
+crbug.com/859169 http/tests/devtools/layers/layer-compositing-reasons.js [ Slow ]
 
 crbug.com/510337 cssom/cssvalue-comparison.html [ Slow ]
 crbug.com/1034030 [ Mac ] http/tests/devtools/resource-har-conversion.js [ Slow ]