Turn on chromium_code (except -Wsign-compare, for now) for pdfium

pdfium.gyp

Index: pdfium.gyp
diff --git a/pdfium.gyp b/pdfium.gyp
index db1c52774a0108d9361c0667d6cc99eac6c5c0af..878879484811963ba4708633651df2b961377b86 100644
--- a/pdfium.gyp
+++ b/pdfium.gyp
@@ -1,5 +1,6 @@
 {
   'variables': {
+    'chromium_code': 1,
     'pdf_use_skia%': 0,
     'pdf_enable_v8%': 1,
     'conditions': [
@@ -42,6 +43,12 @@
     'msvs_disabled_warnings': [
       4005, 4018, 4146, 4333, 4345, 4267
     ],
+    'variables': {
+      'clang_warning_flags': [
+        # TODO(thestig): Fix all instances, remove this, pdfium:29
+        '-Wno-sign-compare',
+      ],
+    },
   },
   'targets': [
     {