Refactoring: Rename isMediaElement to isHTMLMediaElement, toMediaElement to toHTMLMediaElement for consistency, and use toHTMLMediaElement isntead of static_cast.

Type check functions should be named as isClassName, and cast functions should
be named as toClassName.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23122004

git-svn-id: svn://svn.chromium.org/blink/trunk@156279 bbb929c8-8fbe-4397-9dbb-9b2b20218538