Split part of xfa/fgas/BUILD.gn into xfa/fgas/layout/BUILD.gn.

Reduce the scope of the circular dependency with xfa/fde.

Change-Id: Icf710a3c5342cc450b042599ac7a88a95a774a9c
Reviewed-on: https://pdfium-review.googlesource.com/c/47154
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 4834578..3a2cf0a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -279,6 +279,7 @@
       "xfa/fde:unittests",
       "xfa/fgas",
       "xfa/fgas:unittests",
+      "xfa/fgas/layout:unittests",
       "xfa/fwl:unittests",
       "xfa/fxfa:unittests",
       "xfa/fxfa/fm2js:unittests",