DevTools: disable one more test to land front-end changes to color swatch

There's one more refactoring that I'm doing to color and css-var swatches
in DevTools frontend that require one more web test to be disable in order
to land.

Once landed, I'll write a new E2E test in devtools-frontend to cover
what this test was doing instead of just re-enabling it.

Bug: 1137438
Change-Id: I44f8ca212ab7f230c73665befff7c8334d7b91b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505619Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Patrick Brosset <patrick.brosset@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#821761}

third_party/blink/web_tests/TestExpectations

@@ -2472,6 +2472,7 @@ crbug.com/27659 external/wpt/css/css-ruby/ruby-whitespace-002.html [ Failure ]
 crbug.com/1137438 http/tests/devtools/elements/styles-1/color-swatch.js [ Failure Pass ]
 crbug.com/1137438 http/tests/devtools/elements/styles/url-color-swatch.js [ Failure Pass ]
 crbug.com/1137438 http/tests/devtools/sources/color-swatch-position.js [ Failure Pass ]
+crbug.com/1137438 http/tests/devtools/elements/css-variables/color-swatch.js [ Failure Pass ]
 
 # WebRTC: Perfect Negotiation times out in Plan B. This is expected.
 crbug.com/980872 virtual/webrtc-wpt-plan-b/external/wpt/webrtc/RTCPeerConnection-perfect-negotiation.https.html [ Timeout ]