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

experimental/PdfViewer/pdf_viewer_main.cpp

 #include "SkCanvas.h"
 #include "SkCommandLineFlags.h"
 #include "SkDevice.h"
 #include "SkGraphics.h"
 #include "SkImageDecoder.h"
 #include "SkImageEncoder.h"
 #include "SkOSFile.h"
 #include "SkPicture.h"
 #include "SkStream.h"
 #include "SkTypeface.h"
@@ skipping 121 matching lines @@
  */
 static bool make_output_filepath(SkString* path, const SkString& dir,
                                  const SkString& name,
                                  int page) {
     make_filepath(path, dir, name);
     return add_page_and_replace_filename_extension(path, page,
                                                    PDF_FILE_EXTENSION,
                                                    PNG_FILE_EXTENSION);
 }
 
-static void setup_bitmap(SkBitmap* bitmap, int width, int height, SkColor color = SK_ColorWHITE) {
+static void setup_bitmap(SkBitmap* bitmap, int width, int height, SkColor color = SK_ColorTRANSPARENT) {
     bitmap->setConfig(SkBitmap::kARGB_8888_Config, width, height);
 
     bitmap->allocPixels();
     bitmap->eraseColor(color);
 }
 
 /** Write the output of pdf renderer to a file.
  * @param outputDir Output dir.
  * @param inputFilename The skp file that was read.
  * @param renderer The object responsible to write the pdf file.
@@ skipping 179 matching lines @@
     }
 
     return 0;
 }
 
 #if !defined SK_BUILD_FOR_IOS
 int main(int argc, char * const argv[]) {
     return tool_main(argc, (char**) argv);
 }
 #endif