[libFuzzer][Windows] Make fuzzable_proto_library work

Get fuzzable_proto_library working on Windows (and probably Mac) by including
a dummy source in our protobuf_full wrapper.
This causes the library to actually get built.

Bug: 884560, 889889
Change-Id: I4c6a1f8b284448dfdf2845cf3fd9adfd6a4fcb78
Reviewed-on: https://chromium-review.googlesource.com/c/1343243Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609466}

testing/libfuzzer/libprotobuf-mutator.md

@@ -46,9 +46,6 @@ libprotobuf-mutator fuzzer that is even more trivial than
 url_parse_proto_fuzzer.
 
 ## Write a fuzz target for code that accepts protobufs
-*** note
-**Note:** Right now this only works on Linux.
-***
 
 This is almost as easy as writing a standard libFuzzer-based fuzzer. You can
 look at [override_lite_runtime_plugin_test_fuzzer] for an example of a working example of

third_party/libprotobuf-mutator/BUILD.gn

@@ -106,5 +106,8 @@ if (use_libfuzzer) {
     public_deps = [
       "//third_party/protobuf:protobuf_full",
     ]
+    sources = [
+      "dummy.cc",
+    ]
   }
 }

third_party/libprotobuf-mutator/dummy.cc

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Dummy file used to ensure that wrapper libraries get built on non-Linux
+// platforms.

third_party/libprotobuf-mutator/fuzzable_proto_library.gni

@@ -15,9 +15,9 @@ import("//testing/libfuzzer/fuzzer_test.gni")
 import("//third_party/protobuf/proto_library.gni")
 
 template("fuzzable_proto_library") {
-  forward_variables_from(invoker, "*")
   if (use_libfuzzer) {
     proto_library("proto_library_" + target_name) {
+      forward_variables_from(invoker, "*")
       assert(current_toolchain == host_toolchain)
       if (!defined(deps)) {
         deps = []
@@ -42,27 +42,22 @@ template("fuzzable_proto_library") {
       link_target_type = "static_library"
     }
 
-    # Use a static_library for the build target with target_name or
-    # source_set. That way we can also force dependencies to build using
-    # protobuf_full.
+    # By making target a static_library or source_set, we can add protobuf_full
+    # to public_deps.
     target(link_target_type, target_name) {
+      sources = [
+        "//third_party/libprotobuf-mutator/dummy.cc",
+      ]
       public_deps = [
         ":proto_library_" + target_name,
         "//third_party/libprotobuf-mutator:protobuf_full",
       ]
-
-      # Don't bother doing builds for CF if we aren't on Linux because of build
-      # breakages on Mac.
-      # TODO(crbug.com/869035): Figure out what is causing this issue and fix
-      # instead of working around it.
-      if (!is_linux) {
-        all_dependent_configs = [ "//testing/libfuzzer:no_clusterfuzz" ]
-      }
     }
   } else {
     # fuzzable_proto_library should behave like a proto_library when
     # !use_libfuzzer.
     proto_library(target_name) {
+      forward_variables_from(invoker, "*")
     }
   }
 }