PartitionAlloc: Introduce an explicit "recommit" operation.

Previously, recommit was implicit. Now it is an explicit API. The recommit is
currently a no-op on all platforms, but this rearrangement will allow us to
experiment with different commit strategies, particularly on Windows. We're
currently using an uncommon decommit API on Windows, which may be related to
the referenced bug.

BUG=343657

Review URL: https://codereview.chromium.org/271683005

git-svn-id: svn://svn.chromium.org/blink/trunk@174046 bbb929c8-8fbe-4397-9dbb-9b2b20218538