Issue 5131 - [emscripten] Clean separation of bindings code and runtime code

compiled/bindings/runtime_utils.cpp

+#include <emscripten.h>
+
+#include "../intrusive_ptr.h"
+#include "../String.h"
+
+extern "C"
+{
+  void EMSCRIPTEN_KEEPALIVE InitString(DependentString* str,
+      String::value_type* data, String::size_type len)
+  {
+    // String is already allocated on stack, we merely need to call
+    // constructor.
+    new (str) DependentString(data, len);
+  }
+
+  void EMSCRIPTEN_KEEPALIVE DestroyString(OwnedString* str)
+  {
+    // Stack memory will be freed automatically, we need to call
+    // destructor explicitly however.
+    str->~OwnedString();
+  }
+
+  String::size_type EMSCRIPTEN_KEEPALIVE GetStringLength(
+      const String& str)
+  {
+    return str.length();
+  }
+
+  const String::value_type* EMSCRIPTEN_KEEPALIVE GetStringData(
+      const String& str)
+  {
+    return str.data();
+  }
+
+  void EMSCRIPTEN_KEEPALIVE ReleaseRef(ref_counted* ptr)
+  {
+    ptr->ReleaseRef();
+  }
+}