NaCl: Update revision in DEPS, r13921 -> r13931

This pulls in the following Native Client changes:

r13922: (shyamsundarr) cleanup special case compatibility with old validator for avx.
r13923: (mcgrathr) Update nacl_arm_newlib toolchain
r13924: (dschuff) PNaCl: Update LLVM revision in pnacl/COMPONENT_REVISIONS
r13925: (shyamsundarr) fix the check_decoder_test.
r13926: (dschuff) Fix build directory for LLVM regression test runner
r13927: (dschuff) Update revision for PNaCl r13918->r13925
r13928: (dyen) Disable flaky IRT extension thread tests on asan mac bots.
r13931: (shyamsundarr) fix lahf feature detection.

BUG=none
TEST=browser_tests and nacl_integration
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_rel_precise32,linux_nacl_sdk,linux_chromium_dbg

Review URL: https://codereview.chromium.org/662473004

Cr-Commit-Position: refs/heads/master@{#300149}

DEPS

@@ -96,7 +96,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling NaCl
   # and whatever else without interference from each other.
-  'nacl_revision': 'fb58e927098b853a04818c2809b68b55b84af78f', # from svn revision r13921
+  'nacl_revision': 'b1243719f015e44f8031fd104402cfca59accefb', # from svn revision r13931
 }
 
 # Only these hosts are allowed for dependencies in this DEPS file.