Mark http/tests/devtools/elements/shadow/shadow-distribution.js as flaky

Bug: 977015
Change-Id: Ia97e8191e1d8f0a7930baa7783575f79b5b3af7f
Tbr: mjackson@microsoft.com
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675588Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672005}

third_party/blink/web_tests/TestExpectations

@@ -6143,3 +6143,4 @@ crbug.com/978000 inspector-protocol/page/setWebLifecycleState.js [ Pass Timeout
 
 crbug.com/946700 [ Win ] http/tests/devtools/coverage/decorations-after-script-formatter.js [ Pass Timeout ]
 crbug.com/978254 [ Win ] http/tests/devtools/elements/edit/edit-dom-actions-4.js [ Pass Timeout ]
+crbug.com/977015 [ Win ] http/tests/devtools/elements/shadow/shadow-distribution.js [ Pass Timeout ]