Web Animations CSS: Fix interruption of compositor transition waiting for start time

After a compositor animation has been dispatched we await a notification for
it's start time. During this period the start time of the corresponding player
is null but should be treated as the current timeline time.

NOTRY=true
R=shanestephens
TBR=steveblock

BUG=298211

Review URL: https://codereview.chromium.org/80603005

git-svn-id: svn://svn.chromium.org/blink/trunk@162490 bbb929c8-8fbe-4397-9dbb-9b2b20218538