Remove custom changes for protobuf retain_unknown_fields

third_party/protobuf/patches/0001-ignore-option-retain-unknown-fields.patch

---- protobuf-cleaned/src/google/protobuf/descriptor.cc  2015-12-30 13:21:46.000000000 -0800
-+++ protobuf-patched/src/google/protobuf/descriptor.cc  2016-03-31 13:25:26.840024811 -0700
-@@ -5591,6 +5591,18 @@
-                         "\"uninterpreted_option\".");
-   }
- 
-+  // TODO(xyzzyz): remove when all uses in Chromium are removed
-+  if (uninterpreted_option_->name(0).name_part() == "retain_unknown_fields") {
-+    // Chromium patch to protobuf used to introduce a retain_unknown_fields
-+    // option that would make the protobuf_lite runtime retain unknown fields
-+    // just like the protobuf_full would. A newer upstream version of protobuf
-+    // retains these unknown fields even in lite runtime, so the option is no
-+    // longer necessary. Therefore, we ignore this option, and when we remove
-+    // all the occurrences of the option from Chromium, we can remove this
-+    // ignore.
-+    return true;
-+  }
-+
-   const Descriptor* options_descriptor = NULL;
-   // Get the options message's descriptor from the builder's pool, so that we
-   // get the version that knows about any extension options declared in the