NaCl: Update revision in DEPS, r13947 -> r13953

This pulls in the following Native Client changes:

r13948: (mseaborn) Clean up sys/errno.h's #include guard to match the file's name
r13949: (glider) Filter -fsanitize=undefined from cflags when nacl_bootstrap_lib.
r13950: (jvoung) Build gold.nexe with a segment gap, and supply -B, to later support IRT use.
r13952: (jvoung) Add irt_load_optional flag to chrome_main args. Helps w/ making pnacl ld.nexe use IRT.
r13953: (nfullagar) Prep for sdk inclusion - make source more relocatable, so

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/666493004

Cr-Commit-Position: refs/heads/master@{#300592}

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': '2a8cf9cfd9a14e9e721d5e514434cec1d82bf294', # from svn revision r13947
+  'nacl_revision': '6a6d18e28e0b1bba5a408f4940ccbde4f1420d63', # from svn revision r13953
 }
 
 # Only these hosts are allowed for dependencies in this DEPS file.