Pixels class prototype and test (not to be committed).

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index 1d8f585320b9aeaa431f494ad4678e5bdda10f2c..bae4c9e0c357e0cd01e289c20542e38838d35eb6 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -74,6 +74,7 @@ class RawObjectPool;
 class RawOneByteString;
 class RawPatchClass;
 class RawPcDescriptors;
+class RawPixels;
 class RawReceivePort;
 class RawRedirectionData;
 class RawScript;
@@ -449,6 +450,7 @@ class SnapshotReader : public BaseReader {
   RawInstance* NewInstance();
   RawMint* NewMint(int64_t value);
   RawBigint* NewBigint();
+  RawPixels* NewPixels();
   RawTypedData* NewTypedData(intptr_t class_id, intptr_t len);
   RawDouble* NewDouble(double value);
   RawUnresolvedClass* NewUnresolvedClass();
@@ -631,6 +633,7 @@ class SnapshotReader : public BaseReader {
   friend class Namespace;
   friend class ObjectPool;
   friend class PatchClass;
+  friend class Pixels;
   friend class RedirectionData;
   friend class Script;
   friend class Stacktrace;