Store SRTP_PROTECTION_PROFILES as const.

They're small, but they should be read-only. This slightly changes public API
and affects downstream WebRTC code.

Hold on landing this until https://webrtc-codereview.appspot.com/34649004/
rolls into Chromium.

Change-Id: I93cbae20f69d55411d6b1cb62ed7d9a81c83b701
Reviewed-on: https://boringssl-review.googlesource.com/2720
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/stack.h b/include/openssl/stack.h
index 6cde95d..0eeda7f 100644
--- a/include/openssl/stack.h
+++ b/include/openssl/stack.h
@@ -145,7 +145,6 @@
  * STACK_OF:POLICYINFO
  * STACK_OF:POLICYQUALINFO
  * STACK_OF:POLICY_MAPPING
- * STACK_OF:SRTP_PROTECTION_PROFILE
  * STACK_OF:SSL_COMP
  * STACK_OF:STACK_OF_X509_NAME_ENTRY
  * STACK_OF:SXNETID
@@ -168,9 +167,10 @@
  * STACK_OF:X509_VERIFY_PARAM
  * STACK_OF:void
  *
- * We declare STACK_OF(SSL_CIPHER) differently; every SSL_CIPHER is const,
- * so the stack should return const pointers to retain type-checking.
+ * Some stacks contain only const structures, so the stack should return const
+ * pointers to retain type-checking.
  *
+ * CONST_STACK_OF:SRTP_PROTECTION_PROFILE
  * CONST_STACK_OF:SSL_CIPHER */