Oilpan: Adjust FreeList::m_biggestFreeListIndex in ThreadHeap<Header>::merge().

If the split-off heap had free-list entries lager than the biggest one in the
main heap, they were not recycled.

BUG=420515

Review URL: https://codereview.chromium.org/712293002

git-svn-id: svn://svn.chromium.org/blink/trunk@185107 bbb929c8-8fbe-4397-9dbb-9b2b20218538