Seeking media fragment URI before loadeddata event

As per the spec, media fragment URI must seek to start time 
before loadeddata event. This patch makes media element seek 
before the loaddeddata event and thereby removes the 
m_fragmentStartTime.
It also introduces the jumped state which avoids this seek if 
currentTime has been set when readyState is HAVE_NOTHING.

Review URL: https://codereview.chromium.org/539103002

git-svn-id: svn://svn.chromium.org/blink/trunk@182134 bbb929c8-8fbe-4397-9dbb-9b2b20218538