Mark more tests as flaky

A few transitions tests are flaky on Linux (dbg), so marking them as
such.

TBR=kbr@chromium.org

Bug: 769879
Change-Id: Ia7231db555aa53e2627149d97b7f31c80039b13f
Reviewed-on: https://chromium-review.googlesource.com/692614Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505380}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3652,6 +3652,8 @@ crbug.com/760543 [ Linux Release ] virtual/mojo-loading/http/tests/devtools/trac
 
 crbug.com/761891 [ Win7 Linux Debug ] virtual/threaded/animations/transition-and-animation-3.html [ Pass Failure Timeout ]
 crbug.com/769879 [ Linux Debug ] virtual/threaded/transitions/transition-end-event-multiple-02.html [ Pass Failure ]
+crbug.com/769879 [ Linux Debug ] virtual/threaded/transitions/transition-end-event-unprefixed-02.html [ Pass Failure ]
+crbug.com/769879 [ Linux Debug ] virtual/threaded/transitions/transition-end-event-window.html [ Pass Failure ]
 crbug.com/769347 [ Mac ] fast/dom/inert/inert-node-is-uneditable.html [ Failure ]
 
 # Paint-timing tests timeout without threaded-compositing, so skip them.