Fix blink_unittests All/FrameThrottlingTest.GraphicsLayerCollection/1 for LayoutNG

There is a difference between LayoutNG and Legacy about the scrollbar
layer when an iframe is translated to out-of-view. The behavior of
LayoutNG looks correct, but the fact is unrelated to the purpose of the
test, so modified the test to make it pass for both LayoutNG and Legacy,
and still test the graphics layer throttling behavior.

TBR=pdr@chromium.org

Change-Id: I27953ff3a6f3859ef26196f1782ba7e70102b3d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619009Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661222}

third_party/blink/renderer/core/scheduler/frame_throttling_test.cc

@@ -1434,8 +1434,7 @@ TEST_P(FrameThrottlingTest, GraphicsLayerCollection) {
   ASSERT_NE(nullptr, paint_controller);
   // We no longer collect the graphics layers of the iframe and the composited
   // content.
-  EXPECT_EQ(display_item_count - 2,
-            paint_controller->GetDisplayItemList().size());
+  EXPECT_GT(display_item_count, paint_controller->GetDisplayItemList().size());
 
   // Move the child back to the visible viewport.
   frame_element->setAttribute(kStyleAttr,