Moves home card background to HomeCardView.

Moves home card background to HomeCardView.

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

Committed: https://chromium.googlesource.com/chromium/src/+/edc727717b7643cef102702436b9b87238a340db

[oshima, 2014-10-30 19:59:40]

lgtm

[Jun Mukai, 2014-10-31 00:15:21]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2014-10-31 00:16:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/692643003/1

[commit-bot: I haz the power, 2014-10-31 04:29:40]

Patchset 1 (id:??) landed as
https://crrev.com/edc727717b7643cef102702436b9b87238a340db
Cr-Commit-Position: refs/heads/master@{#302219}

[Jun Mukai, 2014-10-31 04:29:42]

Committed patchset #1 (id:1) manually as
edc727717b7643cef102702436b9b87238a340db (presubmit successful).