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No real changes, just adds a DCHECK() and unit test for a minimum
alignment that callers can count on from Map().  Map() is already
returning addresses which are aligned to the platform page size.

Necessary for a media use case in which a wrapped shared memory
object is passed around and is expected to have a certain alignment.

BUG=none
TEST=unittests, try bots.


Review URL: https://chromiumcodereview.appspot.com/10827400

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@152218 0039d316-1c4b-4281-b951-d872f2087c98