Update third_party/protobuf to protobuf-v3.0.0-beta-3

third_party/protobuf/objectivec/Tests/GPBARCUnittestProtos.m

Index: third_party/protobuf/objectivec/Tests/GPBARCUnittestProtos.m
diff --git a/third_party/protobuf/objectivec/Tests/GPBARCUnittestProtos.m b/third_party/protobuf/objectivec/Tests/GPBARCUnittestProtos.m
index d0408869b13bcc388e195e4a4e23dbfd0d19a048..28d2396c2a6209f4f04e04c63995aae8a2695b76 100644
--- a/third_party/protobuf/objectivec/Tests/GPBARCUnittestProtos.m
+++ b/third_party/protobuf/objectivec/Tests/GPBARCUnittestProtos.m
@@ -34,6 +34,11 @@
 
 // Makes sure all the generated headers compile with ARC on.
 
+// The unittest_custom_options.proto extends the messages in descriptor.proto
+// so we build it in to test extending in general. The library doesn't provide
+// a descriptor as it doesn't use the classes/enums.
+#import "google/protobuf/Descriptor.pbobjc.h"
+
 #import "google/protobuf/Unittest.pbobjc.h"
 #import "google/protobuf/UnittestCustomOptions.pbobjc.h"
 #import "google/protobuf/UnittestCycle.pbobjc.h"