Remove stale 'virtual' in DiscardableMemoryAllocator.

DiscardableMemoryAllocator::AshmemRegion used to implement an interface but this
is not the case anymore therefore there is no reason to keep ~AshmemRegion()
virtual (since AshmemRegion is/should never be subclassed either).

TBR=willchan@chromium.org

Review URL: https://codereview.chromium.org/141283006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245520 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245520