Make SkCodec support peek() and read()

src/codec/SkBmpCodec.cpp

Index: src/codec/SkBmpCodec.cpp
diff --git a/src/codec/SkBmpCodec.cpp b/src/codec/SkBmpCodec.cpp
index 580ff25ddd21a70749343fb6cb6de91ffdb5c88d..3302e4f752eb74826b6142d1d3e04535e393f816 100644
--- a/src/codec/SkBmpCodec.cpp
+++ b/src/codec/SkBmpCodec.cpp
@@ -56,12 +56,10 @@ enum BmpInputFormat {
 /*
  * Checks the start of the stream to see if the image is a bitmap
  */
-bool SkBmpCodec::IsBmp(SkStream* stream) {
+bool SkBmpCodec::IsBmp(const void* buffer, size_t bytesRead) {
     // TODO: Support "IC", "PT", "CI", "CP", "BA"
     const char bmpSig[] = { 'B', 'M' };
-    char buffer[sizeof(bmpSig)];
-    return stream->read(buffer, sizeof(bmpSig)) == sizeof(bmpSig) &&
-            !memcmp(buffer, bmpSig, sizeof(bmpSig));
+    return bytesRead >= sizeof(bmpSig) && !memcmp(buffer, bmpSig, sizeof(bmpSig));
 }
 
 /*