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: http://angleproject.googlecode.com/svn/trunk@2186 736b8ea6-26fd-11df-bfd4-992fa37f6226