PartitionAlloc: simplify extent handling a little.

This tidy up is another piece broken out from:
https://codereview.chromium.org/105503009/
(Which is reviewed but reverted.)

TBR=tsepez@chromium.org
BUG=327095

Review URL: https://codereview.chromium.org/132663002

git-svn-id: svn://svn.chromium.org/blink/trunk@164813 bbb929c8-8fbe-4397-9dbb-9b2b20218538