The tests has been flaking for several days now. Example failures:
https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win10/builds/8696/steps/webkit_tests/logs/stdio
https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win10/builds/8699/steps/webkit_tests/logs/stdio
https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win10/builds/8700/steps/webkit_tests/logs/stdio

Diff:

--- E:\b\build\slave\WebKit_Win10\build\layout-test-results\inspector/extensions/extensions-resources-expected.txt
+++ E:\b\build\slave\WebKit_Win10\build\layout-test-results\inspector/extensions/extensions-resources-actual.txt
@@ -59,17 +59,10 @@
     }
 }
 RUNNING TEST: extension_testOpenResourceHandler
-handleOpenResource() invoked [this should only appear once!]: 
-{
-    0 : {
-        getContent : <function>
-        setContent : <function>
-        type : "script"
-        url : .../test-script.js
-    }
-    1 : 3
-}
-RUNNING TEST: extension_testSetResourceContent
-div.test width after stylesheet edited (should be 126): 126
-All tests done.
+Exception while running: InspectorTest.clickOnURL();
+TypeError: Cannot read property 'dispatchEvent' of null
+    at Object.InspectorTest.clickOnURL (evaluateInWebInspector0.js:2075:32)
+    at eval (eval at onEvaluate (evaluateInWebInspector0.js), <anonymous>:1:15)
+    at Object.onEvaluate [as evaluateForTestInFrontEnd] (evaluateInWebInspector0.js:1457:9)
+    at Object._onmessage (file:///E:/b/build/slave/WebKit_Win10/build/src/out/Release/resources/inspector/inspector.js:9977:39)