Revert "Unmap unpinned ashmem regions."

This reverts r232081 since it was a temporary workaround that was needed until
Telemetry supports unpinned ashmem purging before doing memory measurements
(which was added by r235115).

BUG=311633,317626
TBR=willchan

Review URL: https://codereview.chromium.org/64743006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235143 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235143