PartitionAlloc: Remove the 2 GB per-partition address space limit

Depending on allocation patterns, the allocator sometimes uses a lot more
address space than actual committed memory, so the limit -- which was
enforced by crashing -- would sometimes be reached even though actual
memory usage was significantly lower.

Review URL: https://codereview.chromium.org/658113002

git-svn-id: svn://svn.chromium.org/blink/trunk@184081 bbb929c8-8fbe-4397-9dbb-9b2b20218538