SkPDF: add canon assert before adding code that might break it

Motivation: We can write subsets (by page) of pdf documents (but this
in't yet exposed in the public API), but it is a bad idea to mix pages
from multiple documents (de-duping will break).  This assert verifies
that we don't do this by accident in the future.

BUG=skia:3585

Review URL: https://codereview.chromium.org/1037573005

src/pdf/SkPDFDevice.h

@@ -180,6 +180,10 @@ public:
         return *(fFontGlyphUsage.get());
     }
 
+#ifdef SK_DEBUG
+    SkPDFCanon* getCanon() const { return fCanon; }
+#endif  // SK_DEBUG
+
 protected:
     const SkBitmap& onAccessBitmap() SK_OVERRIDE {
         return fLegacyBitmap;

src/pdf/SkPDFDocument.cpp

@@ -70,9 +70,12 @@ bool SkPDFDocument::EmitPDF(const SkTDArray<SkPDFDevice*>& pageDevices,
     if (pageDevices.isEmpty()) {
         return false;
     }
+
     SkTDArray<SkPDFPage*> pages;
     for (int i = 0; i < pageDevices.count(); i++) {
         SkASSERT(pageDevices[i]);
+        SkASSERT(i == 0 ||
+                 pageDevices[i - 1]->getCanon() == pageDevices[i]->getCanon());
         // Reference from new passed to pages.
         pages.push(SkNEW_ARGS(SkPDFPage, (pageDevices[i])));
     }

src/pdf/SkPDFDocument.h

@@ -26,7 +26,6 @@ namespace SkPDFDocument {
  *
  *  @param pageDevices An array of pages, in order.  All pages
  *                     should be created using the same SkPDFCanon.
- *                     TODO(halcanary): ASSERT this condition.
  *  @param SkWStream   The writable output stream to send the PDF to.
  */
 bool EmitPDF(const SkTDArray<SkPDFDevice*>& pageDevices, SkWStream*);