Roll nss 246067:251855

Changes
r251855: Fix compilation of NSPR and NSPR for aarch64

BUG=314720
R=blundell@chromium.org

Review URL: https://codereview.chromium.org/172143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252068 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -56,7 +56,7 @@ vars = {
   # and V8 without interference from each other.
   "webrtc_revision": "5562",
   "jsoncpp_revision": "248",
-  "nss_revision": "246067",
+  "nss_revision": "251855",
   # 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.