PartitionAlloc: support in-place resize of directly mapped allocation

In partitionReallocGeneric(), if both the current size and the new size
are in the direct mapped range, attempt to resize the allocation in place.
When downsizing, this is done by decommitting pages and making them
inaccessible.  When upsizing a previously downsized allocation to a size
not larger than its original size, we make the pages accessible again.

R=cevans@chromium.org
BUG=

Review URL: https://codereview.chromium.org/183113003

git-svn-id: svn://svn.chromium.org/blink/trunk@168633 bbb929c8-8fbe-4397-9dbb-9b2b20218538