[views] Add checkmark throbber to card unmasking prompt

[views] Add checkmark throbber to card unmasking prompt

Also add a new, prettier checkmark asset. (It doesn't appear this is actually used anywhere else, so no other UIs should be affected.)

BUG=none

Committed: https://crrev.com/ad0deae6aeb4b4996203ff40a917e1c88dc8b447
Cr-Commit-Position: refs/heads/master@{#322252}

[Evan Stade, 2015-03-24 19:45:53]

estade@chromium.org changed reviewers:
+ sky@chromium.org

[sky, 2015-03-24 22:08:38]

LGTM

[Evan Stade, 2015-03-24 22:17:54]

The CQ bit was checked by estade@chromium.org

[commit-bot: I haz the power, 2015-03-24 22:18:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1005603003/1

[commit-bot: I haz the power, 2015-03-25 03:47:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-25 03:47:42]

Try jobs failed on following builders:
  linux_chromium_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Evan Stade, 2015-03-25 21:48:02]

The CQ bit was checked by estade@chromium.org

[commit-bot: I haz the power, 2015-03-25 21:48:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1005603003/1

[commit-bot: I haz the power, 2015-03-25 22:30:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-25 22:30:30]

Patchset 1 (id:??) landed as
https://crrev.com/ad0deae6aeb4b4996203ff40a917e1c88dc8b447
Cr-Commit-Position: refs/heads/master@{#322252}