Oilpan: We shouldn't calculate allocationSizeFromSize twice

Currently we calculate allocationSizeFromSize twice in Heap::allocate()
and Heap::outOfLineAllocate(). This CL removes the redundancy.

BUG=420515

Review URL: https://codereview.chromium.org/730443002

git-svn-id: svn://svn.chromium.org/blink/trunk@185355 bbb929c8-8fbe-4397-9dbb-9b2b20218538