Add pdfium_embeddertests to BUILD.gn

This brings BUILD.gn into equivalence with pdfium.gyp.
Blocked on https://codereview.chromium.org/851283006/

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/871453004
diff --git a/BUILD.gn b/BUILD.gn
index 7581d73..4f97e34 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -810,3 +810,21 @@
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ ":pdfium_config", "//build/config/compiler:no_chromium_code" ]
 }
+
+test("pdfium_embeddertests") {
+  sources = [
+    "fpdfsdk/src/fpdfdoc_embeddertest.cpp",
+    "fpdfsdk/src/fpdfview_embeddertest.cpp",
+    "testing/embedder_test.cpp",
+    "testing/embedder_test.h",
+    "testing/fx_string_testhelpers.cpp",
+    "testing/fx_string_testhelpers.h",
+  ]
+  deps = [
+    "//testing/gtest",
+    ":pdfium"
+  ]
+  include_dirs = [ "." ]
+  configs -= [ "//build/config/compiler:chromium_code" ]
+  configs += [ ":pdfium_config", "//build/config/compiler:no_chromium_code" ]
+}