pdfviewer: set the default background to transparent (to match render_picture default), and fix a bug a with corrupt diffs

Review URL: https://codereview.chromium.org/20526002

git-svn-id: http://skia.googlecode.com/svn/trunk@10376 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/experimental/PdfViewer/pdf_viewer_main.cpp b/experimental/PdfViewer/pdf_viewer_main.cpp
index cfea974..d43086f 100644
--- a/experimental/PdfViewer/pdf_viewer_main.cpp
+++ b/experimental/PdfViewer/pdf_viewer_main.cpp
@@ -139,7 +139,7 @@
                                                    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();