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When the shadow layer bounds is smaller than the full shadow image size,
we should crop the corner tiles by reducing aperture values instead of making
the shadow layer invisible.

This allows shadow to draw correctly when window size is very small.

BUG=415514
TEST=visually verify that window shadows are drawn correctly with very small windows.

Review URL: https://codereview.chromium.org/581273002

Cr-Commit-Position: refs/heads/master@{#295728}