Correctly align memory allocations in the case where we need to allocate memory

as far away from the stack as possible, but still as close to the VDSO as we
can.

BUG=none
TEST=run the tests in a tight loop and notice that they no longer randomly fail
Review URL: http://codereview.chromium.org/1807002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@45775 0039d316-1c4b-4281-b951-d872f2087c98