Move JoinString to the base namespace.

Move JoinString to the base namespace.

Change "Separator" from string to StringPIece (most are constants). Remove char versions for symmetry with SplitString.

Update callers who pass empty separators to use a base::StringPiece()

Change chromecast/base/metrics/cast_metrics_helper.cc to not use JoinString at all and just append to the output (code is simpler and faster).

TBR=zelidrag@chromium.org for chromeos

Committed: https://crrev.com/0eabfa001ba06d3c66992b95b69a988ef0633992
Cr-Commit-Position: refs/heads/master@{#338762}

patch from issue 1223153003 at patchset 160001 (http://crrev.com/1223153003#ps160001)

BUG=

Committed: https://crrev.com/d94a2214777c98910f946b6a15258618215545f4
Cr-Commit-Position: refs/heads/master@{#338819}

[brettw, 2015-07-15 02:07:20]

brettw@chromium.org changed reviewers:
+ jam@chromium.org, zelidrag@chromium.org
- zelidrag@chromium.org for chromeos

[brettw, 2015-07-15 02:07:21]

Patch set 1 is the fix for the official build.

[brettw, 2015-07-15 02:07:34]

The CQ bit was checked by brettw@chromium.org

[commit-bot: I haz the power, 2015-07-15 02:08:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230243005/20001

[commit-bot: I haz the power, 2015-07-15 05:19:43]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-15 05:20:47]

Patchset 2 (id:??) landed as
https://crrev.com/d94a2214777c98910f946b6a15258618215545f4
Cr-Commit-Position: refs/heads/master@{#338819}