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We'll soon remove AthenaStartPageView, so AppList independent
components like background should belong to HomeCardView.

This CL also fixes the bug of the visual effect; the spec is
saying that the gradient end should be white even in
the BOTTOM state.

BUG=425717
R=oshima@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/692643003

Cr-Commit-Position: refs/heads/master@{#302219}