Add EME layout test that tests MediaKeys lifetime

Adding a new layout test for EME that allocates a bunch of MediaKey objects and then forces garbage collection.

BUG=341567
TEST=new layout test passes

Review URL: https://codereview.chromium.org/180203002

git-svn-id: svn://svn.chromium.org/blink/trunk@170718 bbb929c8-8fbe-4397-9dbb-9b2b20218538