Revert "[LPM] Improve process for using the plugin for LITE_RUNTIME."

This reverts commit 2f46bdba.

Reason for revert: Commit message incorrect.

Original change's description:
> [LPM] Improve process for using the plugin for LITE_RUNTIME.
> 
> Change protobuf_library to compile everything for the full protobuf runtime
> when use_libfuzzer=true is passed to GN.
> 
> 
> Bug: 860750,861746
> Change-Id: I43d4c51f686895fc8aafb8bdae368815aee4ace2
> Reviewed-on: https://chromium-review.googlesource.com/1128249
> Reviewed-by: Martin Barbella <mbarbella@chromium.org>
> Commit-Queue: Jonathan Metzman <metzman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#577035}

TBR=thakis@chromium.org,mbarbella@chromium.org,metzman@chromium.org

Change-Id: I5bbe8ff3da58383b2b47ebc650906eb90f622cdf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 860750, 861746
Reviewed-on: https://chromium-review.googlesource.com/1145204Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577039}

testing/libfuzzer/libprotobuf-mutator.md

@@ -95,18 +95,28 @@ the full protobuf library, all `.proto` files in Chromium that are used in
 production contain this line: `option optimize_for = LITE_RUNTIME` But this
 line is incompatible with libprotobuf-mutator. Thus, we need to modify the
 `proto_library` build target so that builds when fuzzing are compatible with
-libprotobuf-mutator. To do this, change your `proto_library` to
-`fuzzable_proto_library` (don't worry, this works just like `proto_library` when
-`use_libfuzzer` is `false`) like so:
+libprotobuf-mutator. We do this by using a protobuf compiler plugin,
+`override_lite_runtime_plugin`. Here's what a `proto_library` build target that
+is configured properly looks like:
 
 ```python
-import("//third_party/libprotobuf-mutator/fuzzable_proto_library.gni")
-
-fuzzable_proto_library("my_proto") {
-  ...
+proto_library("my_proto") {
+  ... // These lines (probably) dont need to be changed.
+
+  if (use_libfuzzer && current_toolchain == host_toolchain) {
+    generator_plugin_label =
+      "//third_party/libprotobuf-mutator:override_lite_runtime_plugin"
+    generator_plugin_suffix = ".pb"
+    # The plugin will generate cc, so don't ask for it to be done by protoc.
+    generate_cc = false
+    // If deps is already defined, change "=" to "+=".
+    deps = ["//third_party/libprotobuf-mutator:override_lite_runtime_plugin"]
+  }
 }
 ```
 
+Note that this will not have any affects on the proto_library in production,
+since the plugin is only used when `use_libfuzzer == true`.
 And with that we have completed writing a libprotobuf-mutator fuzz target for
 Chromium code that accepts protobufs.
 

third_party/libprotobuf-mutator/BUILD.gn

 # Copyright 2017 The Chromium Authors. All rights reserved.
 import("//build/config/sanitizers/sanitizers.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
-import("//third_party/libprotobuf-mutator/fuzzable_proto_library.gni")
+import("//third_party/protobuf/proto_library.gni")
 
 config("include_config") {
   include_dirs = [ "src/" ]
@@ -63,9 +63,18 @@ if (current_toolchain == host_toolchain) {
     ]
     public_configs = [ "//third_party/protobuf:protobuf_config" ]
   }
-  # To use the plugin in a proto_library you want to fuzz, change the build
-  # target to fuzzable_proto_library (defined in
-  # //third_party/libprotobuf-mutator/fuzzable_proto_library.gni)
+  # To use the plugin in a proto_library you want to fuzz, add these lines to
+  # the proto_library definition (note the "=" in second to last line in the
+  # comment will need to be changed to "+=" if you have already defined
+  # deps):
+  # if (use_libfuzzer && current_toolchain == host_toolchain) {
+  #   generator_plugin_label =
+  #     "//third_party/libprotobuf-mutator:override_lite_runtime_plugin"
+  #   generator_plugin_suffix = ".pb"
+  #   # The plugin will generate cc, so don't ask for it to be done by protoc.
+  #   generate_cc = false
+  #   deps = ["//third_party/libprotobuf-mutator:override_lite_runtime_plugin"]
+  # }
 }
 
 # The CQ will try building this target without "use_libfuzzer" if it is defined.
@@ -90,21 +99,24 @@ if (use_libfuzzer) {
 }
 
 # Proto library for override_lite_runtime_plugin_test_fuzzer
-fuzzable_proto_library("override_lite_runtime_plugin_test_fuzzer_proto") {
+proto_library("override_lite_runtime_plugin_test_fuzzer_proto") {
   sources = [
     "protoc_plugin/imported.proto",
     "protoc_plugin/imported_publicly.proto",
     "protoc_plugin/test_fuzzer_input.proto",
   ]
-}
 
-# Component that can provide protobuf_full to non-testonly targets
-component("protobuf_full") {
-  # Stop people from using me by accident.
-  if (!use_libfuzzer) {
-    testonly = true
+  # TODO(metzman): Figure out how we can avoid using this toolchain check
+  # (maybe remove compilation from the plugin).
+  if (use_libfuzzer && current_toolchain == host_toolchain) {
+    generator_plugin_label =
+        "//third_party/libprotobuf-mutator:override_lite_runtime_plugin"
+    generator_plugin_suffix = ".pb"
+
+    # The plugin will generate cc, so don't ask for it to be done by protoc.
+    generate_cc = false
+    deps = [
+      "//third_party/libprotobuf-mutator:override_lite_runtime_plugin",
+    ]
   }
-  public_deps = [
-    "//third_party/protobuf:protobuf_full",
-  ]
 }

third_party/libprotobuf-mutator/fuzzable_proto_library.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) {
-      assert(current_toolchain == host_toolchain)
-      if (!defined(deps)) {
-        deps = []
-      }
-      deps +=
-          [ "//third_party/libprotobuf-mutator:override_lite_runtime_plugin" ]
-      generator_plugin_label =
-          "//third_party/libprotobuf-mutator:override_lite_runtime_plugin"
-      generator_plugin_suffix = ".pb"
-
-      # The plugin will generate cc, so don't ask for it to be done by protoc.
-      generate_cc = false
-      generate_python = false
-    }
-
-    # Use a component for the build target with target_name. That way we can
-    # also force dependencies to build using protobuf_full.
-    component(target_name) {
-      public_deps = [
-        ":proto_library_" + target_name,
-        "//third_party/libprotobuf-mutator:protobuf_full",
-      ]
-    }
-  } else {
-    # fuzzable_proto_library should behave like a proto_library when
-    # !use_libfuzzer.
-    proto_library(target_name) {
-    }
-  }
-}