PartitionAlloc: Count direct mapped pages as committed

Extend committed pages counting to direct mapped allocations as well. We
already did adjust the total committed pages size when reallocating a
direct mapped allocation to reflect the difference, but since we didn't do
any counting when initially mapping or when unmapping, this would just
result in accounting errors.

Review URL: https://codereview.chromium.org/649583004

git-svn-id: svn://svn.chromium.org/blink/trunk@184169 bbb929c8-8fbe-4397-9dbb-9b2b20218538