SkPDF: skpdfdocument and skpdfpage use skpdfdevice in a const way

BUG=skia:3585

Review URL: https://codereview.chromium.org/1035513003
diff --git a/src/pdf/SkPDFShader.cpp b/src/pdf/SkPDFShader.cpp
index 97bca73..da55340 100644
--- a/src/pdf/SkPDFShader.cpp
+++ b/src/pdf/SkPDFShader.cpp
@@ -1101,8 +1101,10 @@
             SkNEW_ARGS(SkPDFImageShader, (autoState->detach()));
     imageShader->setData(content.get());
 
+    SkAutoTUnref<SkPDFResourceDict> resourceDict(
+            patternDevice->createResourceDict());
     populate_tiling_pattern_dict(imageShader, patternBBox,
-                                 patternDevice->getResourceDict(), finalMatrix);
+                                 resourceDict.get(), finalMatrix);
 
     imageShader->fShaderState->fImage.unlockPixels();