NaCl: Update revision in DEPS, r6702 -> r6708

This pulls in the following Native Client changes:

r6703: (pdox) Fix the PNacl LLVM merge bot. Always pull and use tip of tree.
r6704: (bauerb) Add support for explicit MIME type mapping in NaCl integration tests.
r6705: (robertm) Update expected values for pnacl spec bots after the changes to the default optimization settings took effect.
r6706: (robertm) A simple csv extractor for benchmarking data collected with the spec2k harness.
r6707: (ncbray) Enable glibc testing on chrome bots.
r6708: (sehr) Get the fix for read sandboxing of 176.gcc committed to mercurial in CL http://codereview.chromium.org/7908014/

BUG=none
TEST=nacl_integration


Review URL: http://codereview.chromium.org/7923015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@101622 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -7,7 +7,7 @@ vars = {
   "webkit_revision": "95343",
   "chromium_git": "http://git.chromium.org/git",
   "swig_revision": "69281",
-  "nacl_revision": "6702",
+  "nacl_revision": "6708",
   # After changing nacl_revision, run 'glient sync' and check native_client/DEPS
   # to update other nacl_*_revision's.
   # TODO(brettw) We should use the "From" syntax to avoid hardcoding the