Remove EventHandller::m_mouseDownWasInSubframe

This patch removes the unused class member in EventHandller.

BUG=N/A
R=rbyers@chromium.org

Review URL: https://codereview.chromium.org/617283002

git-svn-id: svn://svn.chromium.org/blink/trunk@183111 bbb929c8-8fbe-4397-9dbb-9b2b20218538