Remove partitionAlloc() re-entrancy when allocting metadata for super page extents.

We no longer store this type of metadata using our own API. We carve out a space
inside the guard area of the super page extent, which now points to the next
super page extent. In the common single-extent case, we don't fault the metadata
storage page, so we're not throwing away a page.

This change also saves a bit of space in the internal metadata bucket, which no
longer has to cater for storing super page metadata. Previously, this made us
overallocate space for free list metadata, which is a smaller structure.

This change also enables us to revert recent hacky changes to
PartitionAllocTest.FreePageListPageTransitions. I'll take care of that
afterwards.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/30263003

git-svn-id: svn://svn.chromium.org/blink/trunk@160111 bbb929c8-8fbe-4397-9dbb-9b2b20218538