Update source in WebContentsImpl::GetColorProviderColorMaps()

This CL updates the source from which we get the Forced colors color
map in WebContentsImpl::GetColorProviderColorMaps(). Specifically, the
source for the Forced colors color map should be the
DefaultColorProviderSource, which reflects the NativeTheme web instance.
In contrast, the ColorMode color maps(light & dark) will continue to
reflect the corresponding ColorProvider, especially for WebUI, which
tracks the color mode of its associated ColorProvider.

Bug: 40779801
Change-Id: Ibc73f80bd3cb185fc731f89f0a406ca21f98b897
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402446


Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Commit-Queue: Sam Davis Omekara <samomekarajr@microsoft.com>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282116}