Add flag to disable blink gc plugin check

The plugin now has a check to prohibit stl collections of gced types and
members. PDFium intentionally uses such collections and thus can't use
the new check.
This CL adds a flag to BUILD.gn to explicitly disable this check such
that it will not be used in pdfium even when it is enabled by default.

Bug: chromium:1470687
Change-Id: I009068eca560bee4d7d937532f52bb94f0e9b9b1
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/114810
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 42a2c73..4586cc5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -318,6 +318,14 @@
         "-Xclang",
         "blink-gc-plugin",
       ]
+
+      # Disable GC plugin forbidding off-heap collections of GCed:
+      cflags += [
+        "-Xclang",
+        "-plugin-arg-blink-gc-plugin",
+        "-Xclang",
+        "disable-off-heap-collections-of-gced-check",
+      ]
     }
   }
 }