pdfviewer: more prerequired work to have examples working for transparency work items.

experimental/PdfViewer/pdfparser/native/SkNativeParsedPDF.cpp

 #include "SkNativeParsedPDF.h"
 #include "SkPdfNativeTokenizer.h"
 #include "SkPdfBasics.h"
 #include "SkPdfObject.h"
 
 #include <stdio.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 
@@ skipping 105 matching lines @@
 
     if (strcmp((char*)xrefstartKeywordLine, "startxref") != 0) {
         // TODO(edisonn): report/issue
     }
 
     long xrefByteOffset = atol((const char*)xrefByteOffsetLine);
 
     bool storeCatalog = true;
     while (xrefByteOffset >= 0) {
         const unsigned char* trailerStart = readCrossReferenceSection(fFileContent + xrefByteOffset, xrefstartKeywordLine);
-        readTrailer(trailerStart, xrefstartKeywordLine, storeCatalog, &xrefByteOffset, false);
-        storeCatalog = false;
+        xrefByteOffset = -1;
+        if (trailerStart < xrefstartKeywordLine) {
+            readTrailer(trailerStart, xrefstartKeywordLine, storeCatalog, &xrefByteOffset, false);
+            storeCatalog = false;
+        }
     }
 
     // TODO(edisonn): warn/error expect fObjects[fRefCatalogId].fGeneration == fRefCatalogGeneration
     // TODO(edisonn): security, verify that SkPdfCatalogDictionary is indeed using mapper
     // load catalog
 
     if (fRootCatalogRef) {
         fRootCatalog = (SkPdfCatalogDictionary*)resolveReference(fRootCatalogRef);
         if (fRootCatalog->isDictionary() && fRootCatalog->valid()) {
             SkPdfPageTreeNodeDictionary* tree = fRootCatalog->Pages(this);
@@ skipping 399 matching lines @@
     return (SkPdfObject*)ref;
 }
 
 size_t SkNativeParsedPDF::bytesUsed() const {
     return fAllocator->bytesUsed() +
            fContentLength +
            fObjects.count() * sizeof(PublicObjectEntry) +
            fPages.count() * sizeof(SkPdfPageObjectDictionary*) +
            sizeof(*this);
 }