Mark .../devtools/.../decorations-after-script-formatter.js as flaky

Bug: 946700
Change-Id: I97ba94980823b27b40f4cf924835d2b8247e7d0e
No-Try: True
Tbr: caseq@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675004Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671929}

third_party/blink/web_tests/TestExpectations

@@ -6199,3 +6199,5 @@ crbug.com/974710 [ Win7 ] virtual/isolated_world_csp/http/tests/security/isolate
 
 # Sheriff 2019-06-24
 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 ]