[Testing] Clean up CrossThreadStyleValueTest

This CL cleans up the CrossThreadStyleValueTest, which does proper
initialization for the WebThreadSupportingGC, and shut down the thread
at the end of the test, just like what we do in the
PaintWorkletStylePropertyMapTest.

Bug: 895579
Change-Id: I7cf93fe5a192cd221a7f9490982d1a104110a232
Reviewed-on: https://chromium-review.googlesource.com/c/1427605Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625195}

third_party/blink/renderer/core/css/cssom/cross_thread_style_value_test.cc

@@ -18,31 +18,55 @@ namespace blink {
 
 class CrossThreadStyleValueTest : public testing::Test {
  public:
-  static void CheckUnsupportedValue(
+  void ShutDown(WaitableEvent* waitable_event) {
+    DCHECK(!IsMainThread());
+    thread_->ShutdownOnThread();
+    waitable_event->Signal();
+  }
+
+  void ShutDownThread() {
+    WaitableEvent waitable_event;
+    thread_->PostTask(FROM_HERE,
+                      CrossThreadBind(&CrossThreadStyleValueTest::ShutDown,
+                                      CrossThreadUnretained(this),
+                                      CrossThreadUnretained(&waitable_event)));
+    waitable_event.Wait();
+  }
+
+  void CheckUnsupportedValue(
       WaitableEvent* waitable_event,
       std::unique_ptr<CrossThreadUnsupportedValue> value) {
-    EXPECT_EQ(value->value_, "test");
+    DCHECK(!IsMainThread());
+    thread_->InitializeOnThread();
+
+    EXPECT_EQ(value->value_, "Unsupported");
     waitable_event->Signal();
   }
+
+ protected:
+  std::unique_ptr<WebThreadSupportingGC> thread_;
 };
 
 // Ensure that a CrossThreadUnsupportedValue can be safely passed cross
 // threads.
 TEST_F(CrossThreadStyleValueTest, PassUnsupportedValueCrossThread) {
   std::unique_ptr<CrossThreadUnsupportedValue> value =
-      std::make_unique<CrossThreadUnsupportedValue>("test");
+      std::make_unique<CrossThreadUnsupportedValue>("Unsupported");
   DCHECK(value);
 
   // Use a WebThreadSupportingGC to emulate worklet thread.
-  std::unique_ptr<WebThreadSupportingGC> thread = WebThreadSupportingGC::Create(
+  thread_ = WebThreadSupportingGC::Create(
       ThreadCreationParams(WebThreadType::kTestThread));
   WaitableEvent waitable_event;
-  thread->PostTask(
+  thread_->PostTask(
       FROM_HERE,
       CrossThreadBind(&CrossThreadStyleValueTest::CheckUnsupportedValue,
+                      CrossThreadUnretained(this),
                       CrossThreadUnretained(&waitable_event),
                       WTF::Passed(std::move(value))));
   waitable_event.Wait();
+
+  ShutDownThread();
 }
 
 TEST_F(CrossThreadStyleValueTest, CrossThreadUnsupportedValueToCSSStyleValue) {