[wasm] Create HandleScope in OnFinishedStream

{AsyncCompileJob::FinishCompile} assumes that it is called within a
{HandleScope}. This was not the case when it was called at the end of
streaming compilation.

R=clemensh@chromium.org

Bug: chromium:860637
Change-Id: I74508e6cdfc145efb9adc76176abce1ca5713515
Reviewed-on: https://chromium-review.googlesource.com/1172357Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55086}

src/wasm/module-compiler.cc

@@ -2697,6 +2697,7 @@ void AsyncStreamingProcessor::OnFinishedStream(OwnedVector<uint8_t> bytes) {
       // is no code section.
       job_->DoSync<AsyncCompileJob::PrepareAndStartCompile>(true);
     } else {
+      HandleScope scope(job_->isolate_);
       job_->FinishCompile();
     }
   }