Enable test mode for flaky media controls tests

This CL adds test mode to two flaky layout tests to avoid race
conditions causing the flake.

Bug: 835513
Change-Id: I43dfe19852ed064e1016b147f055f99d2d51f969
Reviewed-on: https://chromium-review.googlesource.com/1023043Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552831}

third_party/WebKit/LayoutTests/media/controls/video-enter-exit-fullscreen-while-hovering-shows-controls.html

@@ -11,6 +11,7 @@ video-enter-exit-fullscreen-without-hovering-doesnt-show-controls.html</title>
 <script>
 async_test(function(t) {
   var video = document.querySelector("video");
+  enableTestMode(video);
 
   video.oncanplaythrough = t.step_func(function() {
     video.oncanplaythrough = null;

third_party/WebKit/LayoutTests/media/controls/video-enter-exit-fullscreen-without-hovering-doesnt-show-controls.html

@@ -12,6 +12,7 @@ video-enter-exit-fullscreen-while-hovering-shows-controls.html</title>
 <script>
 async_test(function(t) {
   var video = document.querySelector("video");
+  enableTestMode(video);
 
   video.oncanplaythrough = t.step_func(function() {
     video.oncanplaythrough = null;