Enable cppgc plugins for standalone pdfium builds.

This will catch some cppgc usage errors.

If desired, the plugin can be turned off via the following GN argument
  clang_use_chrome_plugins=false

Bug: pdfium:1671
Change-Id: Ib47c1579e1d863be44db38a63e029442d1bfabb0
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/79171
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index b69dda6..55ca4f2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/clang/clang.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
@@ -47,6 +48,16 @@
   if (is_clang) {
     # Override -Wno-c++11-narrowing.
     cflags += [ "-Wc++11-narrowing" ]
+
+    # Catch misuse of cppgc in XFA.
+    if (pdf_enable_xfa && clang_use_chrome_plugins) {
+      cflags += [
+        "-Xclang",
+        "-add-plugin",
+        "-Xclang",
+        "blink-gc-plugin",
+      ]
+    }
   }
 
   if (!is_win && !is_clang) {