Do not forward declare UIResourceRequest, Identity and IdentityKey.

Clang / libc++ does not support incomplete types in templates.
See: http://clang.llvm.org/compatibility.html#undep_incomplete

This patch will be squashed into the next chromium update.

Change-Id: I46131d53494007548444bc4d158c2208a012ece4
Reviewed-by: Andras Becsi <andras.becsi@digia.com>