Oilpan: Add missing initialization of m_mergePoint on ThreadHeap.

R=oilpan-reviews@chromium.org, zerny@chromium.org
BUG=407098

Review URL: https://codereview.chromium.org/505773002

git-svn-id: svn://svn.chromium.org/blink/trunk@180854 bbb929c8-8fbe-4397-9dbb-9b2b20218538