Avoid synchronous stylesheet update on html import loaded.

third_party/WebKit/Source/web/tests/HTMLImportSheetsTest.cpp

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/dom/Document.h"
+#include "core/html/HTMLLinkElement.h"
+#include "testing/gtest/include/gtest/gtest.h"
+#include "web/tests/sim/SimRequest.h"
+#include "web/tests/sim/SimTest.h"
+
+namespace blink {
+
+class HTMLImportSheetsTest : public SimTest {
+ protected:
+  HTMLImportSheetsTest() { WebView().Resize(WebSize(640, 480)); }
+};
+
+TEST_F(HTMLImportSheetsTest, NeedsActiveStyleUpdate) {
+  SimRequest main_resource("https://example.com/", "text/html");
+  SimRequest import_resource("https://example.com/import.html", "text/html");
+
+  LoadURL("https://example.com/");
+  main_resource.Complete("<link id=link rel=import href=import.html>");
+  import_resource.Complete("<style>div{}</style>");
+
+  EXPECT_TRUE(GetDocument().GetStyleEngine().NeedsActiveStyleUpdate());
+}
+
+}  // namespace blink