Remove EventHandler::m_didStartDrag.

EventHandler maintains a ton of state, and making sure everything is in
sync gets rather complicated. m_didStartDrag was only used as by gesture
code, to detect if it should fall back to default handling.

This patch changes handleDrag()'s CheckDragHysteresis parameter to
DragIniator, which more accurately reflects how it's used. The updated
parameter is also used to determine when default handling is needed,
eliminating the need for the m_didStartDrag bool. Yay!

BUG=none

Review URL: https://codereview.chromium.org/720693003

git-svn-id: svn://svn.chromium.org/blink/trunk@185337 bbb929c8-8fbe-4397-9dbb-9b2b20218538