Most css1/ tests should wait for basebg.gif to be loaded

third_party/WebKit/LayoutTests/css1/resources/base.js

Index: third_party/WebKit/LayoutTests/css1/resources/base.js
diff --git a/third_party/WebKit/LayoutTests/css1/resources/base.js b/third_party/WebKit/LayoutTests/css1/resources/base.js
new file mode 100644
index 0000000000000000000000000000000000000000..af2d8bc7267cdf4ce645244e6bf59f04e2095f6d
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/css1/resources/base.js
@@ -0,0 +1,17 @@
+if (window.testRunner) {
+  // This is a workaroud for an issue that the load event may be dispatched
+  // before loading ../resources/basebg.gif which is linked from
+  // ../resources/base.css.
+  testRunner.waitUntilDone();
+  window.addEventListener('load', () => {
+    const id = setInterval(() => {
+      // Force layout.
+      document.body.offsetTop;
+      if (internals.isLoading('../resources/basebg.gif'))
+        return;
+
+      clearInterval(id);
+      testRunner.notifyDone();
+    }, 50);
+  });
+}