Fix media/track/track-cue-rendering-tree-is-removed-properly.html

The way media/track/track-cue-rendering-tree-is-removed-properly.html is
using assert_throws is no longer valid, so replace it with a different
construct.
Also, <track> does not have a 'mode' content attribute. Using 'default
here seems like it should DTRT.

Bug: 664844
Change-Id: I7cf035dc262160f4748a22f7f92fbc45c2bfe13b
Reviewed-on: https://chromium-review.googlesource.com/756848
Reviewed-by: srirama chandra sekhar <srirama.m@samsung.com>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#514789}
1 file changed