NaCl: Update revision in DEPS, cc52724 -> 2c9f793

This pulls in the following Native Client changes:

044b4c0: (mazda@chromium.org) Implement irt_clear_cache for ARM Non-SFI mode.
ebe73dc: (shinyak@chromium.org) Emit deferred log even when Error is raised.
cd4c4d8: (tfarina@chromium.org) Rename source_prereqs to inputs in the rules.gni file.
4eb5734: (dschuff@chromium.org) Update newlib revision in tools/REVISIONS to 2cc581b
2c9f793: (mseaborn@chromium.org) Cleanup: Move nacl_desc_custom.h into src/public/

BUG=none
TEST=browser_tests and nacl_integration
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_chromium_dbg_32_ng,linux_arm_compile,linux_nacl_sdk_build

Review URL: https://codereview.chromium.org/938823003

Cr-Commit-Position: refs/heads/master@{#317065}

DEPS

@@ -94,7 +94,7 @@ 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': 'cc52724dde60334c60fed1e248f4ebf858db86a5',
+  'nacl_revision': '2c9f79345c54506498a3c2dfbc43920180886e7b',
 }
 
 # Only these hosts are allowed for dependencies in this DEPS file.