Mark some tests flaky

http/tests/w3c/webperf/submission/Intel/user-timing/test_user_timing_mark.html
is flaky on Window Release too.

timeline/tracing/update-layer-tree.html is flaky.

BUG=398295,399507
TBR=alancutter@chromium.org

Review URL: https://codereview.chromium.org/437753002

git-svn-id: svn://svn.chromium.org/blink/trunk@179363 bbb929c8-8fbe-4397-9dbb-9b2b20218538