Update nss_revision to 243960.

Includes the following CLs:

Update third_party/nss to NSS 3.15.4.
https://codereview.chromium.org/105893015/
https://codereview.chromium.org/132203003/

TBR=agl@chromium.org,rsleevi@chromium.org
BUG=331625
TEST=none

Review URL: https://codereview.chromium.org/131973008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244094 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -55,7 +55,7 @@ vars = {
   # and V8 without interference from each other.
   "webrtc_revision": "5300",
   "jsoncpp_revision": "248",
-  "nss_revision": "243121",
+  "nss_revision": "243960",
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling swarming_client
   # and whatever else without interference from each other.