Reland "Fix ScrollNestedLocalNonFastScrollableDiv under ScrollUnification"

This is a reland of 42509254

Original change's description:
> Fix ScrollNestedLocalNonFastScrollableDiv under ScrollUnification
>
> With Scroll Unification, in LayerTreeImpl::DidUpdateScrollOffset there
> are cases where a ScrollUpdate may be mutating a ScrollNode that:
> - has a MainThreadScrollingReason - the main thread needs to repaint it
> each time we scroll.
> - it isn't composited
>
> In this case, when ScrollNode is mutated we don't want to make a
> corresponding change in the associated TransformNode. Instead, we want
> to SetNeedsCommit so that the changed offset in the ScrollNode is
> committed to the main thread, then blink regenerates its side of the
> property trees and cc::Layers.
>
> In this test, we hit the first case. In order for the test to pass we
> need to wait for the main thread to sync.
>
> Bug: 1086625
> Change-Id: Icdc3eb5cc786ebdecd516f3cb0fd58d47077d6ec
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585765
> Reviewed-by: Ken Buchanan <kenrb@chromium.org>
> Reviewed-by: David Bokan <bokan@chromium.org>
> Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#836593}

Bug: 1086625
Change-Id: I053f4eabd5bc62e0959274236919cd53293315ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591088Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838246}

content/browser/site_per_process_hit_test_browsertest.cc

@@ -903,6 +903,9 @@ IN_PROC_BROWSER_TEST_P(SitePerProcessInternalsHitTestBrowserTest,
   rwhv_root->OnScrollEvent(&scroll_event);
   ack_observer.Wait();
 
+  // Wait until renderer's main thread is synced.
+  observer.Wait();
+
   // Verify the div scrolled.
   double div_scroll_top = div_scroll_top_start;
   EXPECT_TRUE(ExecuteScriptAndExtractDouble(