Re-layout grid view and add check marks for selected items.

Re-layout grid view and add check marks for selected items.

Changes are:
- Changed layout of grid view.
- Made thumbnails fill entire item area.
- Use check marks to indicate selected items.

BUG=402344
TEST=run browser_tests

Committed: https://crrev.com/bf8d3d26979fe7c72cc5754050fafedcb6d40f74
Cr-Commit-Position: refs/heads/master@{#313651}

[fukino, 2015-01-28 14:49:22]

fukino@chromium.org changed reviewers:
+ yawano@chromium.org

[fukino, 2015-01-28 14:49:23]

Awano-san, could you take a look? Thanks!

[yawano, 2015-01-29 02:17:12]

lgtm. Thank you!

[fukino, 2015-01-29 02:18:42]

The CQ bit was checked by fukino@chromium.org

[commit-bot: I haz the power, 2015-01-29 02:19:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/885653002/20001

[commit-bot: I haz the power, 2015-01-29 02:37:53]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-29 02:40:31]

Patchset 2 (id:??) landed as
https://crrev.com/bf8d3d26979fe7c72cc5754050fafedcb6d40f74
Cr-Commit-Position: refs/heads/master@{#313651}