SkPDF: replace SkPDFDevice::copyContentToData

Motivation: remove copyToData().  Later we will stop caching
the data alltogether.

BUG=skia:

Review URL: https://codereview.chromium.org/958433003
diff --git a/src/pdf/SkPDFShader.cpp b/src/pdf/SkPDFShader.cpp
index d7605b3..d7a5044 100644
--- a/src/pdf/SkPDFShader.cpp
+++ b/src/pdf/SkPDFShader.cpp
@@ -1110,7 +1110,7 @@
     }
 
     // Put the canvas into the pattern stream (fContent).
-    SkAutoTDelete<SkStream> content(patternDevice->content());
+    SkAutoTDelete<SkStreamAsset> content(patternDevice->content());
 
     SkPDFImageShader* imageShader =
             SkNEW_ARGS(SkPDFImageShader, (canon, autoState->detach()));