Fix applicable scope of Internals::setForcedColorsAndDarkPreferredColorScheme()

Previously the function had a local ColorSchemeHelper which resets
color scheme settings when it destructs, so the forced color flag
only applied to the UpdateAllLifecyclePhasesForTest() in the
function. This causes problems such as
1. If the test runs more document lifecycles (which may be arbitrary
   depending on the scheduler or the test runner), the forced color
   won't apply;
2. Composited scrollbars are not updated with the forced color because
   they are repainted in cc after blink's BeginMainFrame-triggered
   document lifecycle update.

Now store ColorSchemeHelper as an optional field of Internals. After
it's emplaced, it will destruct and restore color scheme settings
when the page unloads.

fast/forms/color-scheme/scrollbar/dynamic-color-scheme-change.html is
modified to include an explicit composited scrolling case, with
rebaselines. The old baseline contained wrong rendering of composited
scrollbars with wrong color schemes.

Bug: 40778657
Change-Id: Ic6c3797198c606231b4f7bfad96729aa39230c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5311652
Reviewed-by: Gaston Rodriguez <gastonr@microsoft.com>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263496}