Tolerate a null scheduler in V8 initialization

Some utility processes need to initailize Blink and V8, and V8
initialization now assumes the main thread provides a scheduler; but
a utility process' main thread does not in fact provide one.

This eliminates the recently introduced assumption of a non-null scheduler,
defaulting to the previous behavior (using the current ThreadTaskRunnerHandle)
when no scheduler is provided.

BUG=679911
R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2624033002
Cr-Commit-Position: refs/heads/master@{#442901}

gin/per_isolate_data.cc

@@ -29,7 +29,8 @@ PerIsolateData::PerIsolateData(
     : isolate_(isolate),
       allocator_(allocator),
       access_mode_(access_mode),
-      task_runner_(task_runner) {
+      task_runner_(
+          task_runner ? task_runner : base::ThreadTaskRunnerHandle::Get()) {
   isolate_->SetData(kEmbedderNativeGin, this);
 }
 

third_party/WebKit/Source/bindings/core/v8/V8Initializer.cpp

@@ -392,9 +392,11 @@ void V8Initializer::initializeMainThread() {
   gin::IsolateHolder::Initialize(gin::IsolateHolder::kNonStrictMode,
                                  v8ExtrasMode, &arrayBufferAllocator);
 
+  // NOTE: Some threads (namely utility threads) don't have a scheduler.
   WebScheduler* scheduler = Platform::current()->currentThread()->scheduler();
-  v8::Isolate* isolate =
-      V8PerIsolateData::initialize(scheduler->timerTaskRunner());
+  v8::Isolate* isolate = V8PerIsolateData::initialize(
+      scheduler ? scheduler->timerTaskRunner()
+                : Platform::current()->currentThread()->getWebTaskRunner());
 
   initializeV8Common(isolate);
 

third_party/WebKit/Source/bindings/core/v8/V8PerIsolateData.cpp

@@ -55,7 +55,7 @@ static void microtasksCompletedCallback(v8::Isolate* isolate) {
 
 V8PerIsolateData::V8PerIsolateData(WebTaskRunner* taskRunner)
     : m_isolateHolder(WTF::makeUnique<gin::IsolateHolder>(
-          taskRunner->toSingleThreadTaskRunner())),
+          taskRunner ? taskRunner->toSingleThreadTaskRunner() : nullptr)),
       m_stringCache(WTF::wrapUnique(new StringCache(isolate()))),
       m_hiddenValue(V8HiddenValue::create()),
       m_privateProperty(V8PrivateProperty::create()),