Make sure that selection is cleared after tapping.

The problem is that Selection is not getting cleared 
after tapping of gesture. This was affected by 
https://codereview.chromium.org/507533002, 
because initialization for m_selectionInitiationState was 
skipped in case of gesture. 
This patch makes m_selectionInitiationState be initialized
after dispatching mousedown event for gesture tapping.

BUG=429967
R=rbyers@chromium.org
TEST=LayoutTests/fast/events/touch/gesture/gesture-tap-reset-selection-range.html

Review URL: https://codereview.chromium.org/705733002

git-svn-id: svn://svn.chromium.org/blink/trunk@185410 bbb929c8-8fbe-4397-9dbb-9b2b20218538