Use InputTypeNames more in MediaControlElements

Use existing AtomicStrings instead of a string literals
that need to be converted to them.

Review URL: https://codereview.chromium.org/716873002

git-svn-id: svn://svn.chromium.org/blink/trunk@185154 bbb929c8-8fbe-4397-9dbb-9b2b20218538