Add HEAP_EXPORT to ThreadHeap::outOfLineAllocate to fix linker errors on GCC/component build

TBR=ager@chromium.org

Review URL: https://codereview.chromium.org/126883004

git-svn-id: svn://svn.chromium.org/blink/trunk@164627 bbb929c8-8fbe-4397-9dbb-9b2b20218538