SkPDF: add assert for that SkWStream behaves.

src/doc/SkDocument_PDF.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDocument.h"
 #include "SkPDFCanon.h"
 #include "SkPDFDevice.h"
@@ skipping 183 matching lines @@
     */
 
     // Build font subsetting info before proceeding.
     SkPDFSubstituteMap substitutes;
     perform_font_subsetting(pageDevices, &substitutes);
 
     SkPDFObjNumMap objNumMap;
     if (objNumMap.addObject(docCatalog.get())) {
         docCatalog->addResources(&objNumMap, substitutes);
     }
-    size_t baseOffset = SkToOffT(stream->bytesWritten());
+    size_t baseOffset = stream->bytesWritten();
     emit_pdf_header(stream);
     SkTDArray<int32_t> offsets;
     for (int i = 0; i < objNumMap.objects().count(); ++i) {
         SkPDFObject* object = objNumMap.objects()[i];
-        offsets.push(SkToS32(stream->bytesWritten() - baseOffset));
+        size_t offset = stream->bytesWritten();
+        // This assert checks that size(pdf_header) > 0 and that
+        // the output stream correctly reports bytesWritten().
+        SkASSERT(offset > baseOffset);
+        offsets.push(SkToS32(offset - baseOffset));
         SkASSERT(object == substitutes.getSubstitute(object));
         SkASSERT(objNumMap.getObjectNumber(object) == i + 1);
         stream->writeDecAsText(i + 1);
         stream->writeText(" 0 obj\n");  // Generation number is always 0.
         object->emitObject(stream, objNumMap, substitutes);
         stream->writeText("\nendobj\n");
     }
     int32_t xRefFileOffset = SkToS32(stream->bytesWritten() - baseOffset);
 
     // Include the zeroth object in the count.
@@ skipping 126 matching lines @@
 
 SkDocument* SkDocument::CreatePDF(const char path[], SkScalar dpi) {
     SkFILEWStream* stream = SkNEW_ARGS(SkFILEWStream, (path));
     if (!stream->isValid()) {
         SkDELETE(stream);
         return NULL;
     }
     auto delete_wstream = [](SkWStream* stream, bool) { SkDELETE(stream); };
     return SkNEW_ARGS(SkDocument_PDF, (stream, delete_wstream, dpi));
 }