Add schannel webrtc_base build using a new use_schannel gyp variable.

R=henrike@webrtc.org, thorcarpenter@google.com

Review URL: https://webrtc-codereview.appspot.com/28409004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7141 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/build/common.gypi

@@ -118,6 +118,11 @@
     # https://gcc.gnu.org/wiki/LinkTimeOptimization
     'use_lto%': 0,
 
+    # Defer ssl perference to that specified through sslconfig.h instead of
+    # choosing openssl or nss directly.  In practice, this can be used to
+    # enable schannel on windows.
+    'use_legacy_ssl_defaults%': 0,
+
     'conditions': [
       ['build_with_chromium==1', {
         # Exclude pulse audio on Chromium since its prerequisites don't require