Substitute ASSERT_UNUSED() for ASSERT() to resolve a compiler warning
that the argument platformLayer is unused. The argument platformLayer
is only used in the asserted condition, which isn't compiled in a
release build; => the argument is unused in a release build.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::parseAttribute):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162473 268f45cc-cd09-0410-ab3c-d52691b4dbfc