Added definition of ALLOW_WASM to CMakeLists.

This change is required to run "toolchain_build_pnacl.py --cmake", since
without it, ALLOW_WASM will be undefined and throw a compilation error.

BUG=None
R=eholk@chromium.org

Review URL: https://codereview.chromium.org/1897323002 .
1 file changed