Add a method to read a stream without advancing it.

src/core/SkStream.cpp

Index: src/core/SkStream.cpp
diff --git a/src/core/SkStream.cpp b/src/core/SkStream.cpp
index 426f5569e887fe83e4aee8b38c412587abfa1fe2..ad67a0ba7db43759c8c6f9e03ff2f25bc0ac43ca 100644
--- a/src/core/SkStream.cpp
+++ b/src/core/SkStream.cpp
@@ -367,6 +367,20 @@ size_t SkMemoryStream::read(void* buffer, size_t size) {
     return size;
 }
 
+bool SkMemoryStream::peek(void* buffer, size_t size) const {
+    SkASSERT(buffer != NULL);
+    const size_t position = fOffset;
+    if (size > fData->size() - position) {
+        // The stream is not large enough to satisfy this request.
+        return false;
+    }
+    SkMemoryStream* nonConstThis = const_cast<SkMemoryStream*>(this);
+    SkDEBUGCODE(const size_t bytesRead =) nonConstThis->read(buffer, size);
+    SkASSERT(bytesRead == size);
+    nonConstThis->fOffset = position;
+    return true;
+}
+
 bool SkMemoryStream::isAtEnd() const {
     return fOffset == fData->size();
 }