Allow overriding GN build flags

build_overrides/pdfium.gni

Index: build_overrides/pdfium.gni
diff --git a/build_overrides/pdfium.gni b/build_overrides/pdfium.gni
new file mode 100644
index 0000000000000000000000000000000000000000..32e11c5bfc8998d3e3efe2515a6fd1a53ac691d2
--- /dev/null
+++ b/build_overrides/pdfium.gni
@@ -0,0 +1,5 @@
+# Copyright 2016 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.
+
+import("../pdfium.gni")

[Nico, 2016/04/27 16:43:29]

From what I understand, this file should set the pdfium variables controlling
xfa and stuff to the values it should have in Chromium. The pdfium version of
this file should set them to the value they should have in pdfium.

So having this file be empty looks wrong to me.

[dsinclair, 2016/04/27 16:49:59]

So, the values we want in PDFium are the default values set in pdfium.gni. I had
to add this file because the other BUILD.gn files reference
build_overrides/pdfium.gni so it will get picked up in the Chrome build.

If the PDFium files referenced ../pdfium.gni and the Chrome files used
build_overrides/pdfium.gni I ended up with different flags set in different
places (i.e. the third_party/BUILD.gn had pdf_enable_xfa=false when other files
had it as true.)