Revert "Update protobuf to 3.6.0. Fixes #5439"

This reverts commit 6fd4e0edd4de22dec9eda13dc0b29214f2ca117e.
diff --git a/third_party/protobuf/3.4.0/gmock.BUILD b/third_party/protobuf/3.4.0/gmock.BUILD
new file mode 100644
index 0000000..b1ae15a
--- /dev/null
+++ b/third_party/protobuf/3.4.0/gmock.BUILD
@@ -0,0 +1,28 @@
+cc_library(
+    name = "gtest",
+    srcs = [
+        "googletest/src/gtest-all.cc",
+        "googlemock/src/gmock-all.cc",
+    ],
+    hdrs = glob([
+        "**/*.h",
+        "googletest/src/*.cc",
+        "googlemock/src/*.cc",
+    ]),
+    includes = [
+        "googlemock",
+        "googletest",
+        "googletest/include",
+        "googlemock/include",
+    ],
+    linkopts = ["-pthread"],
+    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "gtest_main",
+    srcs = ["googlemock/src/gmock_main.cc"],
+    linkopts = ["-pthread"],
+    visibility = ["//visibility:public"],
+    deps = [":gtest"],
+)