Don't fall back to default incognito colors and tints when using a custom theme.
Don't fall back to default incognito colors and tints when using a
custom theme.

BUG=596136

Review URL: https://codereview.chromium.org/1813393002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#383895}

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