Make SkCodec support peek() and read()

src/codec/SkJpegCodec.cpp

Index: src/codec/SkJpegCodec.cpp
diff --git a/src/codec/SkJpegCodec.cpp b/src/codec/SkJpegCodec.cpp
index 6e2fc84ec8b58983bdc2b5ff5311171149f862f9..6ea13b7639dd560d9e06d5b490f26be267616e46 100644
--- a/src/codec/SkJpegCodec.cpp
+++ b/src/codec/SkJpegCodec.cpp
@@ -23,11 +23,9 @@ extern "C" {
     #include "jpeglib.h"
 }
 
-bool SkJpegCodec::IsJpeg(SkStream* stream) {
+bool SkJpegCodec::IsJpeg(const void* buffer, size_t bytesRead) {
     static const uint8_t jpegSig[] = { 0xFF, 0xD8, 0xFF };
-    char buffer[sizeof(jpegSig)];
-    return stream->read(buffer, sizeof(jpegSig)) == sizeof(jpegSig) &&
-            !memcmp(buffer, jpegSig, sizeof(jpegSig));
+    return bytesRead >= 3 && !memcmp(buffer, jpegSig, sizeof(jpegSig));
 }
 
 bool SkJpegCodec::ReadHeader(SkStream* stream, SkCodec** codecOut,