Use double-precision for times in AudioParam methods

The start times, end times, time constants, and durations are all double-precision in the spec:
https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#AudioParam

BUG=none
TEST=these APIs and scheduling are covered extensively by current tests
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/18456002

git-svn-id: svn://svn.chromium.org/blink/trunk@154248 bbb929c8-8fbe-4397-9dbb-9b2b20218538