RunPendingTasks required after stylesheet load.

When BlockHTMLParserOnStyleSheets is enabled, two of the lazy image
loading tests requires RunPendingTasks to finish parsing after the
stylesheet loading is completed.

Bug: 891767
Change-Id: I82acfe201b1988a6f10e5793868073b63708aad2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2011805Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733763}

third_party/blink/renderer/core/html/lazy_load_image_observer_test.cc

@@ -544,6 +544,7 @@ TEST_P(LazyLoadImagesParamsTest, NearViewport) {
       kViewportHeight + GetLoadingDistanceThreshold() - 100));
 
   css_resource.Complete("img { width: 50px; height: 50px; }");
+  test::RunPendingTasks();
 
   Vector<char> full_image = ReadTestImage();
   ASSERT_LT(2048U, full_image.size());
@@ -677,6 +678,7 @@ TEST_P(LazyLoadImagesParamsTest, FarFromViewport) {
       kViewportHeight + GetLoadingDistanceThreshold() + 100));
 
   css_resource.Complete("img { width: 50px; height: 50px; }");
+  test::RunPendingTasks();
 
   Compositor().BeginFrame();
   test::RunPendingTasks();