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These aren't used by the default theme, which renders programmatically
or with colors. (For example, the ntp background image was a 42x42
white square, but we don't need it because we set the bg color to
white.)

I left the IDR_s in place because they're still needed to identify
custom theme images. We could replace them with a C++ enum, but since
they occupy the same namespace as other themed/not-themed images (ints),
leaving them here seems the best way to avoid potential collisions.

BUG=522168

Review-Url: https://codereview.chromium.org/2568403004
Cr-Commit-Position: refs/heads/master@{#438989}