Add more documentation for View::OnNativeThemeChanged.
Add more documentation for View::OnNativeThemeChanged.

As requested in https://codereview.chromium.org/1967203003/

BUG=none

Review-Url: https://codereview.chromium.org/1971023003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393176}

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