Bug 958916: Update NSS to NSS 3.15.5 beta 2 (NSS_3_15_5_BETA2) plus private patch for bug 950129, CLOBBER, r=me

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extra : rebase_source : a0a28ba744ecf81510a4c5bec3a353e4fe275f5a
extra : amend_source : e8b89e0560fc75de5557b8e4543269363f417132
Bug 890994: ALPN support for Gecko, r=briansmith