Add a method to read a stream without advancing it.

src/utils/SkFrontBufferedStream.cpp

Index: src/utils/SkFrontBufferedStream.cpp
diff --git a/src/utils/SkFrontBufferedStream.cpp b/src/utils/SkFrontBufferedStream.cpp
index f23b1f99c4c6da1c98f8b947647ce977421546e8..0351c3a11e01e9d4ad2bdbbd03dff765fb6efd6c 100644
--- a/src/utils/SkFrontBufferedStream.cpp
+++ b/src/utils/SkFrontBufferedStream.cpp
@@ -16,6 +16,8 @@ public:
 
     size_t read(void* buffer, size_t size) override;
 
+    size_t peek(void* buffer, size_t size) override;
+
     bool isAtEnd() const override;
 
     bool rewind() override;
@@ -155,6 +157,20 @@ size_t FrontBufferedStream::readDirectlyFromStream(char* dst, size_t size) {
     return bytesReadDirectly;
 }
 
+size_t FrontBufferedStream::peek(void* dst, size_t size) {
+    if (fOffset >= fBufferSize) {
+        // This stream is not able to buffer any more.
+        return 0;
+    }
+    // Keep track of the offset so we can return to it.
+    const size_t start = fOffset;
+    // Do not read more than we can buffer.
+    const size_t bytesToRead = SkTMin(size, fBufferSize - start);
+    const size_t bytesRead = this->read(dst, bytesToRead);
+    fOffset = start;
+    return bytesRead;
+}
+
 size_t FrontBufferedStream::read(void* voidDst, size_t size) {
     // Cast voidDst to a char* for easy addition.
     char* dst = reinterpret_cast<char*>(voidDst);