PNaCl: Update libc++ revision in pnacl/COMPONENT_REVISIONS
PNaCl: Update libc++ revision in pnacl/COMPONENT_REVISIONS

This pulls in the following libc++ change:

518a2ff: (dschuff@chromium.org) Merge remote-tracking branch 'origin/upstream/master'

This brings in a merge to upstream tip of tree.
In particular it brings in r208942 which is needed for the
offsetof macro to work with clang after r207606, which
changed cstddef (see comments on those revisions).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3623
TEST= PNaCl toolchain trybots
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/460873002

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@13606 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client