Split pdfium_embeddertests sources.

Move foo_embeddertest.cpp to the same BUILD.gn file as foo.cpp.
Set up dependencies to make pdfium_embeddertests pass gn check.

Change-Id: If1fe67c261960c35c344c1047950cd7fdb32c4a1
Reviewed-on: https://pdfium-review.googlesource.com/c/43997
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxcodec/BUILD.gn b/core/fxcodec/BUILD.gn
index 1c68956..e09b7a6 100644
--- a/core/fxcodec/BUILD.gn
+++ b/core/fxcodec/BUILD.gn
@@ -147,3 +147,10 @@
     ]
   }
 }
+
+pdfium_embeddertest_source_set("embeddertests") {
+  sources = [
+    "codec/fx_codec_embeddertest.cpp",
+  ]
+  pdfium_root_dir = "../../"
+}