Add sync experiment flag for wallet datatype.

Add sync experiment flag for wallet datatype.

If either the sync experiment OR the command line flag is present, the feature is enabled. (Enabling the feature also gives the user the ability to control it via a separate pref.)

BUG=

Committed: https://crrev.com/4e911e45e27caf4a92dc71a60c32b20e77a17430
Cr-Commit-Position: refs/heads/master@{#314407}

[Evan Stade, 2015-01-27 02:25:08]

estade@chromium.org changed reviewers:
+ brettw@chromium.org, kevinliu@google.com, zea@chromium.org

[Evan Stade, 2015-01-27 02:25:08]

We currently have a command line flag that controls whether the feature is
enabled. That is of course set by the client. Where does this flag get set?

[Evan Stade, 2015-02-03 01:59:48]

updated, PTAL

[Nicolas Zea, 2015-02-03 18:34:39]

sync LGTM

[brettw, 2015-02-03 20:29:38]

lgtm

[Evan Stade, 2015-02-03 20:31:35]

The CQ bit was checked by estade@chromium.org

[commit-bot: I haz the power, 2015-02-03 20:32:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/875243003/20001

[commit-bot: I haz the power, 2015-02-03 21:08:47]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-03 21:09:49]

Patchset 2 (id:??) landed as
https://crrev.com/4e911e45e27caf4a92dc71a60c32b20e77a17430
Cr-Commit-Position: refs/heads/master@{#314407}