Update net/third_party/nss to NSS_3_15_5_BETA2.

net/third_party/nss/README.chromium

Index: net/third_party/nss/README.chromium
===================================================================
--- net/third_party/nss/README.chromium	(revision 245705)
+++ net/third_party/nss/README.chromium	(working copy)
@@ -1,6 +1,6 @@
 Name: Network Security Services (NSS)
 URL: http://www.mozilla.org/projects/security/pki/nss/
-Version: 3.15.4
+Version: 3.15.5 Beta 2
 Security Critical: Yes
 License: MPL 2
 License File: NOT_SHIPPED
@@ -11,7 +11,7 @@
 The same module appears in crypto/third_party/nss (and third_party/nss on some
 platforms), so we don't repeat the license file here.
 
-The snapshot was updated to the hg tag: NSS_3_15_4_RTM
+The snapshot was updated to the hg tag: NSS_3_15_5_BETA2
 
 Patches:
 
@@ -62,9 +62,6 @@
   * Update Chromium-specific code for TLS 1.2.
     patches/tls12chromium.patch
 
-  * Add the Application Layer Protocol Negotiation extension.
-    patches/alpn.patch
-
   * Add Chromium-specific code to detect AES GCM support in the system NSS
     libraries at run time. Remove this patch when all system NSS packages are
     NSS 3.15 or later.
@@ -78,12 +75,8 @@
     patches/cachelocks.patch
     https://bugzilla.mozilla.org/show_bug.cgi?id=764646
 
-  * In the case that a ClientHello record is between 256 and 511 bytes long,
-    add an extension to make it 512 bytes. This works around a bug in F5
-    terminators.
-    patches/paddingextension.patch
+  * Always add an extension to a ClientHello to make it 512 bytes.
     patches/paddingextensionall.patch
-    https://bugzilla.mozilla.org/show_bug.cgi?id=944157
 
   * Support the Certificate Transparency (RFC 6962) TLS extension
     signed_certificate_timestamp (client only).