ui: Stop converting Rect to RectF implicitly in native_theme_win.cc
ui: Stop converting Rect to RectF implicitly in native_theme_win.cc

TBR=sky
BUG=342848

Review URL: https://codereview.chromium.org/1334553002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#347983}

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