[wasm] Delete all compilation units even after compilation errors

FinishCompilationUnits used the assumption that FinishCompilationUnit
only return null if there is no compilation unit left to be finished.
This assumption was wrong though, because also a compilation error can
cause the result to be null. Therefore I switched to use the function
index as a new indicator.

BUG=chromium:709174

Change-Id: I3e9689fd71b8364422e1c74404921df2799191aa
Reviewed-on: https://chromium-review.googlesource.com/471347
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44468}

src/wasm/wasm-module.cc

@@ -396,19 +396,18 @@ class CompilationHelper {
   void FinishCompilationUnits(std::vector<Handle<Code>>& results,
                               ErrorThrower* thrower) {
     while (true) {
-      int func_index = 0;
-      MaybeHandle<Code> result = FinishCompilationUnit(thrower, &func_index);
-      if (result.is_null()) break;
-      results[func_index] = result.ToHandleChecked();
+      int func_index = -1;
+      Handle<Code> result = FinishCompilationUnit(thrower, &func_index);
+      if (func_index < 0) break;
+      results[func_index] = result;
     }
   }
 
-  MaybeHandle<Code> FinishCompilationUnit(ErrorThrower* thrower,
-                                          int* func_index) {
+  Handle<Code> FinishCompilationUnit(ErrorThrower* thrower, int* func_index) {
     compiler::WasmCompilationUnit* unit = nullptr;
     {
       base::LockGuard<base::Mutex> guard(&result_mutex_);
-      if (executed_units_.empty()) return {};
+      if (executed_units_.empty()) return Handle<Code>::null();
       unit = executed_units_.front();
       executed_units_.pop();
     }
@@ -2892,16 +2891,15 @@ class AsyncCompileJob {
     HandleScope scope(isolate_);
     if (failed_) return true;  // already failed
 
-    int func_index = 0;
+    int func_index = -1;
     ErrorThrower thrower(isolate_, nullptr);
-    MaybeHandle<Code> result =
-        helper_->FinishCompilationUnit(&thrower, &func_index);
+    Handle<Code> result = helper_->FinishCompilationUnit(&thrower, &func_index);
     if (thrower.error()) {
       RejectPromise(isolate_, context_, &thrower, module_promise_);
       failed_ = true;
     } else {
-      code_table_->set(func_index + module_->num_imported_functions,
-                       *(result.ToHandleChecked()));
+      DCHECK(func_index >= 0);
+      code_table_->set(func_index + module_->num_imported_functions, *(result));
     }
     if (failed_ || --outstanding_units_ == 0) {
       // All compilation units are done. We still need to wait for the