Web Animations: Allow player start time to be set after adding to timeline

When a Player's start time is not set its effective current time before
drift will be treated as 0.

This will be used to support the delayed start of compositor animations
and could be used as a basis for play/playNow.

Review URL: https://codereview.chromium.org/61743018

git-svn-id: svn://svn.chromium.org/blink/trunk@162063 bbb929c8-8fbe-4397-9dbb-9b2b20218538
getConfig() is deprecated, simply use config().

BUG=None
TEST=None, no functional changes
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/61933004

git-svn-id: svn://svn.chromium.org/blink/trunk@162064 bbb929c8-8fbe-4397-9dbb-9b2b20218538