Mark media-source/idlharness test as slow

The test often times out on the waterfall.

Tbr: chcunningham@chromium.org
Change-Id: I47cc2c795f0ff4163e952d1cce971388b7a67760
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773524Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690852}

third_party/blink/web_tests/external/wpt/media-source/idlharness.window.js

 // META: script=/resources/WebIDLParser.js
 // META: script=/resources/idlharness.js
+// META: timeout=long
 
 // https://w3c.github.io/media-source/