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This change:
1) Adds 4 new properties to CaptionStyle.
2) Adds caption_style_win.cc that contains a Windows implementation for
   CaptionStyle::FromSystemSettings().
3) Adds a new feature flag to enable Windows system styling.
4) Follows the same pattern as the earlier change here:
   https://chromium-review.googlesource.com/c/chromium/src/+/1418399

Next steps:
Add support for background opacity, window opacity, and window color

Bug: 897730

Change-Id: Iefb0092c32b4fdb3d4d6412a2e21b9bdf018887f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1482140Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Rahul Singh <rahsin@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#638316}