Mark some devtools/elements/styles tests as flaky on Win/Linux

http/tests/devtools/elements/styles-3/styles-computed-trace.js
http/tests/devtools/elements/styles-3/styles-add-new-rule-colon.js
http/tests/devtools/elements/styles-2/force-pseudo-state.js

TBR=luoe@chromium.org

Bug: 994008, 994034, 994027
Change-Id: I47a3cb1c8c6d3355d28e54a56b05a2eca6207f6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760796Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689419}

third_party/blink/web_tests/TestExpectations

@@ -6496,3 +6496,8 @@ crbug.com/914134 [ Mac ] vr/requestPresent_resolve_then_reject.html [ Failure ]
 crbug.com/996219 [ Win ] fast/text/emoji-vertical-origin-visual.html [ Failure ]
 
 crbug.com/996597 [ Win ] css2.1/t0905-c5525-flthw-00-c-g.html [ Failure ]
+
+# Sheriff 2019-08-22
+crbug.com/994008 [ Linux Win ] http/tests/devtools/elements/styles-3/styles-computed-trace.js [ Pass Failure ]
+crbug.com/994034 [ Linux Win ] http/tests/devtools/elements/styles-3/styles-add-new-rule-colon.js [ Pass Failure ]
+crbug.com/994027 [ Linux Win ] http/tests/devtools/elements/styles-2/force-pseudo-state.js [ Pass Failure ]