Remove MainThreadScrollingReason::kNoScrollingLayer

Previously ScrollTree::GetMainThreadRepaintReasons() returned
kNoScrollingLayer for a ScrollNode with !is_composited &&
!main_thread_scrolling_reasons, which happened only if blink omitted
painting the transform node for a non-composited scroller [1].
0.02% (Android [1]) and 0.4% (Windows [2]) of scrolls were with
this reason.

The reason caused the scrolls to be blocked by the main thread,
but that was unnecessary because the scrollers were not visible.

Now add ScrollTree::ShouldRealizeScrollsOnMain() to replace some
!CanRealizeScrollsOnCompositor() callsites (both functions return
false if the transform node is not painted), to avoid main-thread
scroll update for invisible scrollers.

This also prepares for RasterInducingScroll which will introduce
CanRealizeScrollsOnPendingTree() (and CanRealizeScrollsOnCompositor()
will become CanRealizeScrollOnActiveTree()), which will apply for
scroll nodes with !is_composited && !main_thread_scrolling_reasons.
This CL makes it possible to repurpose the condition for scroll
nodes applicable for RasterInducingScroll.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/5368550/3/third_party/blink/renderer/platform/graphics/compositing/property_tree_manager.cc#618
[2] https://uma.googleplex.com/p/chrome/timeline_v2?sid=d5728bceab6ac2663d0474d53a08711e
[3] https://uma.googleplex.com/p/chrome/timeline_v2?sid=8eae2fe4139db6dc62d78be9bc1448de

Bug: 40256365, 40517276
Change-Id: I0b0e3b6488b980accd9a89caabcd809ea0f12169
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5368550


Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1273497}