pdfviewer: set backdround to transparent, and add an app to chop transparent content in png files.

gyp/pdfviewer.gyp

 # GYP file to build pdfviewer.
 #
 # To build on Linux:
 #  ./gyp_skia pdfviewer.gyp && make pdfviewer
 #
 {
   'includes': [
     'apptype_console.gypi',
   ],
   'targets': [
@@ skipping 81 matching lines @@
         '../experimental/PdfViewer',
         '../experimental/PdfViewer/pdfparser',
         '../experimental/PdfViewer/pdfparser/autogen',
         '../experimental/PdfViewer/pdfparser/native',
         '../experimental/PdfViewer/pdfparser/native/autogen',
       ],
       'dependencies': [
         'core.gyp:core',
         'flags.gyp:flags',
         'libpdfviewer',
+        'chop_transparency',
+      ],
+    },
+    {
+      'target_name': 'chop_transparency',
+      'type': 'executable',
+      'sources': [
+        '../experimental/PdfViewer/chop_transparency_main.cpp',
+      ],
+      'include_dirs': [
+        # For SkBitmapHasher.h
+        '../src/utils/',
+      ],
+      'dependencies': [
+        'skia_lib.gyp:skia_lib',
+        'flags.gyp:flags',
       ],
     },
   ],
 }
 
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