Refactor kColorForcedColor* colors to kColorCSSSystem*

This CL refactors the kColorForcedColors* color ids, corresponding
filenames and functions to a more generic kColorCSSSystem*. These
color ids are defined to replace and deprecate the existing
system_colors map stored in the NativeTheme class. Initially, an
incorrect assumption was made that system colors are only used in
forced colors mode hence the previous name definitions. However, this
assumption is wrong because system colors can also be used in light
mode for Windows (See: LayoutThemeWin::SystemColor()). With the
deprecation of the system_colors map stored in the NativeTheme, it is
important that the nomenclature of these color ids reflect what they
are used for.

More specifically, this change:
1.) Renames color ids prefixed with kColorForcedColors* to
kColorCSSSystem
2.) Renames the files forced_colors* to css_system_color*
3.) Renames function AddForcedColorsColorMixer to
AddCSSSystemColorMixer
4.) Renames function AddSystemForcedColorsToMixer to
AddNativeSystemColorsToMixer
5.) Adds the default native system colors mixer when in Light mode

In subsequent changes, these ids will be added to RendererColorMap,
allowing them to be used in Blink. This will enable the use of color
providers to retrieve system colors in Blink, ultimately leading to the
deprecation of the UpdateSystemColorInfo() pipeline.

Bug: 1430181
Change-Id: I69e2958e63742f2a4391dbd652d30e959afcfe68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5338343
Commit-Queue: Sam Davis Omekara <samomekarajr@microsoft.com>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1269241}