Switch SkPDFStream's internal storage from SkStream to SkData

Motivation: This makes SkPDFStream thread-safe for two threads
serializing it at once, since a SkStream has an internal position.

Updated SkPDFFont, SkPDFGraphicState, and SkPDFPage's use of
SkPDFStream to use the SkData constructor rather than the SkStream
constructor (saving a memcpy).

BUG=skia:2683

Committed: https://skia.googlesource.com/skia/+/c1dfa14b645ae274780f026dd86c9b633fbdad06

R=mtklein@google.com, djsollen@google.com, rmistry@google.com, robertphillips@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/340783013
diff --git a/src/images/SkImageDecoder_libbmp.cpp b/src/images/SkImageDecoder_libbmp.cpp
index f9dd247..7b87e40 100644
--- a/src/images/SkImageDecoder_libbmp.cpp
+++ b/src/images/SkImageDecoder_libbmp.cpp
@@ -12,7 +12,7 @@
 #include "SkImageDecoder.h"
 #include "SkScaledBitmapSampler.h"
 #include "SkStream.h"
-#include "SkStreamHelpers.h"
+#include "SkStreamPriv.h"
 #include "SkTDArray.h"
 
 class SkBMPImageDecoder : public SkImageDecoder {
@@ -99,7 +99,7 @@
     // Allocated space used to hold the data.
     SkAutoMalloc storage;
     // Byte length of all of the data.
-    const size_t length = CopyStreamToStorage(&storage, stream);
+    const size_t length = SkCopyStreamToStorage(&storage, stream);
     if (0 == length) {
         return 0;
     }