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/pdfium.gni b/pdfium.gni
index ec71db9..b85ac74 100644
--- a/pdfium.gni
+++ b/pdfium.gni
@@ -17,4 +17,7 @@
 
   # Build PDFium against skia (experimental) rather than agg.
   pdf_use_skia = false
+
+  # Build PDFium standalone
+  pdf_is_standalone = false
 }