Mark resource-timing-sizes-tags.html as flaky

Bug=786235
TBR=ricea@chromium.org

Change-Id: I8f8540e451473ec7fdc0b4176f651e527cc0d599
Reviewed-on: https://chromium-review.googlesource.com/776253Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517252}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4110,3 +4110,7 @@ crbug.com/667560 virtual/mojo-loading/http/tests/devtools/elements/elements-pane
 # Sheriff failures 2017-11-16
 crbug.com/785980 [ Win10 ] virtual/mojo-loading/http/tests/devtools/network/network-xhr-same-url-as-main-resource.js [ Failure Pass ]
 crbug.com/785980 [ Win10 ] http/tests/devtools/network/network-xhr-same-url-as-main-resource.js [ Failure Pass ]
+
+# Sheriff failures 2017-11-17
+crbug.com/786235 http/tests/misc/resource-timing-sizes-tags.html [ Failure Pass ]
+crbug.com/786235 virtual/mojo-loading/http/tests/misc/resource-timing-sizes-tags.html [ Failure Pass ]