Use proper clang plugin name in GN build.

build/config/clang/BUILD.gn

Index: build/config/clang/BUILD.gn
diff --git a/build/config/clang/BUILD.gn b/build/config/clang/BUILD.gn
index d083e86c11b452113f8b04bbe02be8aa45385151..46bf985e7dc3aa1297aabf342fa706404503bb0d 100644
--- a/build/config/clang/BUILD.gn
+++ b/build/config/clang/BUILD.gn
@@ -6,12 +6,19 @@ config("find_bad_constructs") {
   cflags = [
     "-Xclang", "-load",
     "-Xclang",
+  ]
 
-    # TODO(brettw) express this in terms of a relative dir from the output.
-    # for now, assume the output dir is two levels deep under the source
-    # (like "out/Debug").
-    "../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib",
+  if (is_mac) {
+    cflags += rebase_path(
+      "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib",
+      ".", root_build_dir)
+  } else if (is_linux) {
+    cflags += rebase_path(
+      "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so",
+      ".", root_build_dir)
+  }
 
+  cflags += [
     "-Xclang", "-add-plugin",
     "-Xclang", "find-bad-constructs",
   ]