SkPDF: allow PDF module to be disabled in DM and SampleApp

gyp/pdf.gyp

Index: gyp/pdf.gyp
diff --git a/gyp/pdf.gyp b/gyp/pdf.gyp
index 693d3307da14d5ad531874be6a0ba0246632abd3..1e69d9164ec2d819fb994bb90c1cdcc8e7bcb23b 100644
--- a/gyp/pdf.gyp
+++ b/gyp/pdf.gyp
@@ -6,6 +6,13 @@
 {
   'targets': [
     {
+      'target_name': 'nopdf',
+      'type': 'static_library',
+      'dependencies': [ 'skia_lib.gyp:skia_lib', ],
+      'sources': [ '<(skia_src_path)/doc/SkDocument_PDF_None.cpp', ],
+      'defines': [ 'SK_SUPPORT_PDF=0', ],
+    },
+    {
       'target_name': 'pdf',
       'product_name': 'skia_pdf',
       'type': 'static_library',
@@ -46,6 +53,7 @@
         ],
       ],
       'direct_dependent_settings': {
+        'defines': [ 'SK_SUPPORT_PDF=1', ],
         'include_dirs': [
           '../include/core',  # SkDocument.h
         ],