Bug 1405511 - Re-enable 3DES on nightly builds r=keeler

In bug 1386754 we disabled 3DES after determining that it had a similar-ish
usage level as RC4. We gathered compatibility reports and telemetry for the last
two months and see that while 3DES usage is fairly low, it is the only
ciphersuite available for a variety of websites, including many government
systems.

3DES, while legacy, is not known to be insecure. Therefore, we're going to call
this experiment complete, use the collected WebCompat issues from Bug 1386908
for future reference, and re-enable 3DES.

MozReview-Commit-ID: 3lY1zHLNO9l

--HG--
extra : rebase_source : ecb51c6dbc6862991083b1f46920d86d7480582f

security/manager/ssl/security-prefs.js

@@ -28,12 +28,7 @@ pref("security.ssl3.dhe_rsa_aes_128_sha", true);
 pref("security.ssl3.dhe_rsa_aes_256_sha", true);
 pref("security.ssl3.rsa_aes_128_sha", true);
 pref("security.ssl3.rsa_aes_256_sha", true);
-// Deprecate 3DES on nightly builds, Bug 1386754
-#ifdef RELEASE_OR_BETA
 pref("security.ssl3.rsa_des_ede3_sha", true);
-#else
-pref("security.ssl3.rsa_des_ede3_sha", false);
-#endif
 
 pref("security.content.signature.root_hash",
      "97:E8:BA:9C:F1:2F:B3:DE:53:CC:42:A4:E6:57:7E:D6:4D:F4:93:C2:47:B4:14:FE:A0:36:81:8D:38:23:56:0E");