Add rappor::SampleString for consisntecy with domain+registry case

Add rappor::SampleString for consisntecy with domain+registry case

This patch adds a function to sample a string to RAPPOR, in the same manner
as rappor::SampleDomainAndRegistryFromGURL(). This makes callers look more
consistent regardless whether it records a string or a domain+registry of
a URL. Suggested in a review comment of this CL:
https://codereview.chromium.org/1014543003/#msg48

BUG=461671, 429901
Committed: https://crrev.com/4e5674cf918801089ca56baf5f2fb1142e943e18
Cr-Commit-Position: refs/heads/master@{#324475}

[kojii, 2015-04-08 18:03:44]

kojii@chromium.org changed reviewers:
+ holte@chromium.org

[kojii, 2015-04-08 18:03:45]

PTAL.

[Steven Holte, 2015-04-08 20:36:14]

On 2015/04/08 18:03:45, koji wrote:
> PTAL.

Maybe add the type param and just change this to rappor::SampleString?

[kojii, 2015-04-09 01:00:55]

Thank you for the quick review, renamed.

PTAL.

[Steven Holte, 2015-04-09 17:34:05]

lgtm

[kojii, 2015-04-09 18:00:33]

The CQ bit was checked by kojii@chromium.org

[commit-bot: I haz the power, 2015-04-09 18:01:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1070863002/20001

[commit-bot: I haz the power, 2015-04-09 18:16:51]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-09 18:17:32]

Patchset 2 (id:??) landed as
https://crrev.com/4e5674cf918801089ca56baf5f2fb1142e943e18
Cr-Commit-Position: refs/heads/master@{#324475}