Mark media browser tests as preload=auto.

Speculative fix to see if flake is related to preload=metadata
suspend/resume issues. This is the classic cause of this type of
problem, so it seems likely we've had another regression here.

BUG=986021
TEST=none
TBR=liberato

Change-Id: I44a7c9428a3b123ea9d17dbe6b6a25a028d845b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790262Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694355}

media/test/data/player.html

@@ -204,6 +204,7 @@ function RunTest() {
 
   // Starts the player.
   player.loop = loop;
+  player.preload = 'auto';
   player.src = media_url;
   Log('Starting by calling play()');
   player.play().catch(function(error) {