Standalone GN build.

This CL setups up a standalone GN build. You'll need to set gn args similar to:

use_goma = true
clang_use_chrome_plugins = false
pdf_enable_xfa = true
pdf_enable_v8 = true
pdf_is_standalone = true

The third_party/pymock files are needed to make git cl upload work correctly.

BUG=pdfium:106

Review URL: https://codereview.chromium.org/1904563002
diff --git a/BUILD.gn b/BUILD.gn
index c54d57f..3ecc9a9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1695,7 +1695,7 @@
     "//testing/gmock",
     "//testing/gtest",
   ]
-  include_dirs = []
+  include_dirs = [ "testing/gmock/include" ]
   if (pdf_enable_v8) {
     sources += [
       "fpdfsdk/javascript/public_methods_embeddertest.cpp",
@@ -1715,3 +1715,12 @@
   }
   configs += [ ":pdfium_config" ]
 }
+
+if (pdf_is_standalone) {
+  source_set("samples") {
+    testonly = true
+    deps = [
+      "//samples",
+    ]
+  }
+}