[iOS] Upstream //ios/chrome/browser/ui/commands changes

[iOS] Upstream //ios/chrome/browser/ui/commands changes

ShowSigninCommand is a command to perform a sign in operation.

Modernize other commands to use "instancetype", "NS_DESIGNATED_INITIALIZER",
"NS_UNAVAILABLE" for the initializers and "@synthetize" for the properties.

Remove unused ShowAccountsSettingsCommand.

BUG=429756
Committed: https://crrev.com/a92ac8eb225f2c3d9fe0a5fde04850136ee9ee07
Cr-Commit-Position: refs/heads/master@{#329627}

[sdefresne, 2015-05-12 13:52:19]

Patchset #1 (id:1) has been deleted

[sdefresne, 2015-05-12 13:53:27]

sdefresne@chromium.org changed reviewers:
+ jif@chromium.org

[jif, 2015-05-13 09:15:29]

lgtm

[sdefresne, 2015-05-13 09:47:31]

On 2015/05/13 09:15:29, jif wrote:
> lgtm

Thank you for the review. Please do not send to CQ as I'm waiting for the
cleanup to be reviewed downstream. I will update this CL to be in sync with
downstream CL once it lands.

[sdefresne, 2015-05-13 13:19:57]

On 2015/05/13 09:47:31, sdefresne wrote:
> On 2015/05/13 09:15:29, jif wrote:
> > lgtm
> 
> Thank you for the review. Please do not send to CQ as I'm waiting for the
> cleanup to be reviewed downstream. I will update this CL to be in sync with
> downstream CL once it lands.

Downstream CL landed, sending to CQ.

[sdefresne, 2015-05-13 13:20:01]

The CQ bit was checked by sdefresne@chromium.org

[sdefresne, 2015-05-13 13:20:02]

The patchset sent to the CQ was uploaded after l-g-t-m from jif@chromium.org
Link to the patchset: https://codereview.chromium.org/1124173004/#ps40001
(title: "Fix compilation")

[commit-bot: I haz the power, 2015-05-13 13:20:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1124173004/40001

[commit-bot: I haz the power, 2015-05-13 13:24:23]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2015-05-13 13:25:10]

Patchset 2 (id:??) landed as
https://crrev.com/a92ac8eb225f2c3d9fe0a5fde04850136ee9ee07
Cr-Commit-Position: refs/heads/master@{#329627}