Handle SkStream::rewind properly.

include/core/SkStream.h:
Update documentation to state that rewinding a stream at the beginning
should return true. This is important because our decoders fail if
rewind returns false, assuming that the stream is not at the beginning.

src/images/SkImageDecoder_libpng.cpp:
If rewind fails, call png_error.

src/images/SkImageDecoder_libwebp.cpp:
If rewind fails, report an error and return false.

src/images/SkImageRef.cpp:
If rewind fails report an error and return false.
FIXME: Need to handle flattening properly. Should I perhaps move
writeStream into SkOrderedWriteBuffer?

src/images/SkJpegUtility.cpp:
Report a jpeg error on failure to rewind.

BUG=https://b.corp.google.com/issue?id=8432093
R=bungeman@google.com, djsollen@google.com, reed@google.com

Review URL: https://codereview.chromium.org/22861028

git-svn-id: http://skia.googlecode.com/svn/trunk@10977 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/images/SkImageRef.cpp b/src/images/SkImageRef.cpp
index 299166c..bd4a739 100644
--- a/src/images/SkImageRef.cpp
+++ b/src/images/SkImageRef.cpp
@@ -107,7 +107,10 @@
 
     SkASSERT(fBitmap.getPixels() == NULL);
 
-    fStream->rewind();
+    if (!fStream->rewind()) {
+        SkDEBUGF(("Failed to rewind SkImageRef stream!"));
+        return false;
+    }
 
     SkImageDecoder* codec;
     if (fFactory) {
@@ -184,6 +187,15 @@
     buffer.writeUInt(fConfig);
     buffer.writeInt(fSampleSize);
     buffer.writeBool(fDoDither);
-    fStream->rewind();
-    buffer.writeStream(fStream, fStream->getLength());
+    // FIXME: Consider moving this logic should go into writeStream itself.
+    // writeStream currently has no other callers, so this may be fine for
+    // now.
+    if (!fStream->rewind()) {
+        SkDEBUGF(("Failed to rewind SkImageRef stream!"));
+        buffer.write32(0);
+    } else {
+        // FIXME: Handle getLength properly here. Perhaps this class should
+        // take an SkStreamAsset.
+        buffer.writeStream(fStream, fStream->getLength());
+    }
 }