Add a ResourceConstraint for the embedder to specify that V8 is running on a memory constrained device.

This enables us to specialize certain operations such that we limit memory
usage on low-memory devices, without reducing performance on devices which
are not memory constrained.

BUG=chromium:280984
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23464022

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16608 ce2b1a6d-e550-0410-aec6-3dcde31c8c00