[gin] Add a v8::Platform implementation to allow for v8 posting tasks

gin/per_isolate_data.cc

Index: gin/per_isolate_data.cc
diff --git a/gin/per_isolate_data.cc b/gin/per_isolate_data.cc
index b3f24abe00d97dcd23ca52133a5fd0d4acdc6740..99c928cdd8ea1e02b0699c2243c176e537899286 100644
--- a/gin/per_isolate_data.cc
+++ b/gin/per_isolate_data.cc
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "base/logging.h"
+#include "base/message_loop/message_loop_proxy.h"
 #include "gin/per_isolate_data.h"
 #include "gin/public/gin_embedders.h"
 
@@ -18,7 +19,9 @@ namespace gin {
 
 PerIsolateData::PerIsolateData(Isolate* isolate,
                                ArrayBuffer::Allocator* allocator)
-    : isolate_(isolate), allocator_(allocator) {
+    : isolate_(isolate),
+      allocator_(allocator),
+      message_loop_proxy_(base::MessageLoopProxy::current()) {
   isolate_->SetData(kEmbedderNativeGin, this);
 }