Do not use non-standard "precision" attribute in media controls.

precision="float" is equivalent to step="any".
We're counting "precision" usage. Using it in media controls has made the data
useless.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/47893002

git-svn-id: svn://svn.chromium.org/blink/trunk@160688 bbb929c8-8fbe-4397-9dbb-9b2b20218538