[wasm] Remove redundant allocation of CompilationHelper

Inside the CompilationHelper, we were creating another compilation
helper to execute sequential or parallel compilation.
I don't see the reason to do so.

R=ahaas@chromium.org

Change-Id: Ib2c4486296a8f923e7e38620879c02963fff7d60
Reviewed-on: https://chromium-review.googlesource.com/497754Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45157}

src/wasm/wasm-module.cc

@@ -562,18 +562,16 @@ class CompilationHelper {
         ->AddSample(static_cast<int>(module_->functions.size()));
 
     if (!lazy_compile) {
-      CompilationHelper helper(isolate_, module_);
       size_t funcs_to_compile =
           module_->functions.size() - module_->num_imported_functions;
       if (!FLAG_trace_wasm_decoder && FLAG_wasm_num_compilation_tasks != 0 &&
           funcs_to_compile > 1) {
         // Avoid a race condition by collecting results into a second vector.
         std::vector<Handle<Code>> results(temp_instance.function_code);
-        helper.CompileInParallel(&module_env, results, thrower);
+        CompileInParallel(&module_env, results, thrower);
         temp_instance.function_code.swap(results);
       } else {
-        helper.CompileSequentially(&module_env, temp_instance.function_code,
-                                   thrower);
+        CompileSequentially(&module_env, temp_instance.function_code, thrower);
       }
       if (thrower->error()) return {};
     }