SkPDF: skpdfdocument and skpdfpage use skpdfdevice in a const way

src/pdf/SkPDFDocument.cpp

Index: src/pdf/SkPDFDocument.cpp
diff --git a/src/pdf/SkPDFDocument.cpp b/src/pdf/SkPDFDocument.cpp
index 57eab950edb987bbe0c2649183b08c17920d8dbf..077af79b15414cef87357880111c75f232b6f3e6 100644
--- a/src/pdf/SkPDFDocument.cpp
+++ b/src/pdf/SkPDFDocument.cpp
@@ -65,7 +65,7 @@ static void emit_pdf_footer(SkWStream* stream,
     stream->writeText("\n%%EOF");
 }
 
-bool SkPDFDocument::EmitPDF(const SkTDArray<SkPDFDevice*>& pageDevices,
+bool SkPDFDocument::EmitPDF(const SkTDArray<const SkPDFDevice*>& pageDevices,
                             SkWStream* stream) {
     if (pageDevices.isEmpty()) {
         return false;