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This change uses the native_theme and scrollbar_theme properties to set
correct dark mode styling for scrollbars.

Bug: 929098
Change-Id: Ia06047ca3ec8571cf25acaf8de4731f1ccc16459
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148283Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Sam Sebree <sasebree@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#760579}