Schedule frames for popup widget for web tests with compositor thread

When we run web tests with --enable-thread-compositing we have
a compositor thread available.
The code in web_page_popup_impl.cc assumes all web tests are run
single-threaded.
In single-threaded web tests, the owner frame tree runs the
composite step for the popup.
If --enable-thread-compositing is used allow the popup to run
the composite step.

      virtual/scroll-unification/fast/forms/suggestion-picker,
      virtual/scroll-unification/fast/time

Bug: 1144434
TEST: virtual/scroll-unification/fast/forms/select-popup,
Change-Id: I969f825fd6fc331f1e1ed682b98b3e18caeb4f5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636296
Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846249}

third_party/blink/renderer/core/exported/web_page_popup_impl.cc

@@ -73,6 +73,7 @@
 #include "third_party/blink/renderer/platform/instrumentation/tracing/trace_event.h"
 #include "third_party/blink/renderer/platform/instrumentation/use_counter.h"
 #include "third_party/blink/renderer/platform/keyboard_codes.h"
+#include "third_party/blink/renderer/platform/scheduler/public/thread.h"
 #include "third_party/blink/renderer/platform/text/text_direction.h"
 #include "third_party/blink/renderer/platform/web_test_support.h"
 #include "third_party/blink/renderer/platform/widget/frame_widget.h"
@@ -173,9 +174,12 @@ class PagePopupChromeClient final : public EmptyChromeClient {
     // do not wish to actually do anything.
     if (popup_->closing_)
       return;
-    if (WebTestSupport::IsRunningWebTest()) {
+
+    // When the renderer has a compositor thread we need to follow the
+    // normal code path.
+    if (WebTestSupport::IsRunningWebTest() && !Thread::CompositorThread()) {
       // In single-threaded web tests, the owner frame tree runs the composite
-      // step for the popup. Popup widgets don't runany composite step on their
+      // step for the popup. Popup widgets don't run any composite step on their
       // own. And we don't run popup tests with a compositor thread, so no need
       // to check for that.
       Document& opener_document =

third_party/blink/web_tests/VirtualTestSuites

@@ -789,7 +789,10 @@
   },
   {
     "prefix": "scroll-unification",
-    "bases": ["fast/scrolling/scrollbars"],
+    "bases": ["fast/scrolling/scrollbars",
+              "fast/forms/select-popup",
+              "fast/forms/suggestion-picker",
+              "fast/forms/time"],
     "args": ["--enable-features=ScrollUnification",
              "--enable-threaded-compositing"]
   },