[offthread] Disable off-thread compiles for block coverage

Off-thread compilation currently doesn't support adding DebugInfo to a
SharedFunctionInfo, so it also doesn't support adding coverage info.

For now, disable off-thread finalization entirely when block coverage
is enabled. In the future we most likely want to add support for this.

Bug: chromium:1011762
Change-Id: I7fdcee48f427d0f7061e90602d9d7557a98e54bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2126911
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67354}

src/codegen/compiler.cc

@@ -1199,7 +1199,9 @@ BackgroundCompileTask::BackgroundCompileTask(ScriptStreamingData* streamed_data,
       streamed_data->source_stream.get(), streamed_data->encoding));
   info_->set_character_stream(std::move(stream));
 
-  finalize_on_background_thread_ = FLAG_finalize_streaming_on_background;
+  // TODO(leszeks): Add block coverage support to off-thread finalization.
+  finalize_on_background_thread_ =
+      FLAG_finalize_streaming_on_background && !flags_.block_coverage_enabled();
   if (finalize_on_background_thread()) {
     off_thread_isolate_ =
         std::make_unique<OffThreadIsolate>(isolate, info_->zone());