Call standard libjpeg/libjpeg-turbo APIs

BUG=skia:

Review URL: https://codereview.chromium.org/1322623004
diff --git a/src/codec/SkJpegCodec.h b/src/codec/SkJpegCodec.h
index ce4fe56..38e41e7 100644
--- a/src/codec/SkJpegCodec.h
+++ b/src/codec/SkJpegCodec.h
@@ -116,6 +116,9 @@
     bool nativelyScaleToDimensions(uint32_t width, uint32_t height); 
 
     SkAutoTDelete<JpegDecoderMgr> fDecoderMgr;
+    // We will save the state of the decompress struct after reading the header.
+    // This allows us to safely call onGetScaledDimensions() at any time.
+    const int                     fReadyState;
     
     friend class SkJpegScanlineDecoder;