Test field trial group with startswith rather than equals.

BUG=webrtc:7266

Review-Url: https://codereview.webrtc.org/2717973005
Cr-Commit-Position: refs/heads/master@{#16915}
diff --git a/webrtc/pc/peerconnection.cc b/webrtc/pc/peerconnection.cc
index c3ec0b2..48ffebe 100644
--- a/webrtc/pc/peerconnection.cc
+++ b/webrtc/pc/peerconnection.cc
@@ -2510,8 +2510,8 @@
   // by experiment.
   if (configuration.disable_ipv6) {
     portallocator_flags &= ~(cricket::PORTALLOCATOR_ENABLE_IPV6);
-  } else if (webrtc::field_trial::FindFullName("WebRTC-IPv6Default") ==
-             "Disabled") {
+  } else if (webrtc::field_trial::FindFullName("WebRTC-IPv6Default")
+                 .find("Disabled") == 0) {
     portallocator_flags &= ~(cricket::PORTALLOCATOR_ENABLE_IPV6);
   }