[d8] Add handlescope within message loop

D8 workers were leaking all messages due to a missing HandleScope
within the message loop. Add one to fix leaks.

R=verwaest@chromium.org
BUG=v8:9524

Change-Id: I2f2f6ba0b8c58bf38bf68b7464dffdf1d826a29d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1724212Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62992}

src/d8/d8.cc

@@ -2699,6 +2699,7 @@ void Worker::ExecuteInThread() {
                   .ToLocalChecked();
           if (onmessage->IsFunction()) {
             Local<Function> onmessage_fun = Local<Function>::Cast(onmessage);
+            SealHandleScope shs(isolate);
             // Now wait for messages
             while (true) {
               in_semaphore_.Wait();
@@ -2708,6 +2709,7 @@ void Worker::ExecuteInThread() {
                 break;
               }
               v8::TryCatch try_catch(isolate);
+              HandleScope scope(isolate);
               Local<Value> value;
               if (Shell::DeserializeValue(isolate, std::move(data))
                       .ToLocal(&value)) {