[wasm] Add a magic word and a version number to the binary.

src/wasm/encoder.cc

Index: src/wasm/encoder.cc
diff --git a/src/wasm/encoder.cc b/src/wasm/encoder.cc
index d80a27533884401ae845ec185b385225c56361c7..a6b2f43983e9d1952f0274a7aa5f55e6d244f435 100644
--- a/src/wasm/encoder.cc
+++ b/src/wasm/encoder.cc
@@ -461,6 +461,8 @@ struct Sizes {
 WasmModuleIndex* WasmModuleWriter::WriteTo(Zone* zone) const {
   Sizes sizes = {0, 0};
 
+  sizes.Add(2 * sizeof(uint32_t), 0);  // header
+
   sizes.Add(1, 0);
   sizes.Add(kDeclMemorySize, 0);
 
@@ -495,6 +497,10 @@ WasmModuleIndex* WasmModuleWriter::WriteTo(Zone* zone) const {
   byte* header = buffer;
   byte* body = buffer + sizes.header_size;
 
+  // -- emit magic -------------------------------------------------------------
+  EmitUint32(&header, kWasmMagic);
+  EmitUint32(&header, kWasmVersion);
+
   // -- emit memory declaration ------------------------------------------------
   EmitUint8(&header, kDeclMemory);
   EmitUint8(&header, 16);  // min memory size