Validate allocation size in PoolAllocator. Note that I am planning to get rid of PoolAllocator entirely. BUG=crbug 179654 Review URL: https://codereview.appspot.com/8662046 git-svn-id: https://angleproject.googlecode.com/svn/branches/es3proto@2333 736b8ea6-26fd-11df-bfd4-992fa37f6226