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).

[email protected] 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=

Review URL: https://codereview.chromium.org/1230243005

Cr-Commit-Position: refs/heads/master@{#338819}
diff --git a/chrome/browser/net/firefox_proxy_settings.cc b/chrome/browser/net/firefox_proxy_settings.cc
index 2b23e01d5..203d9557 100644
--- a/chrome/browser/net/firefox_proxy_settings.cc
+++ b/chrome/browser/net/firefox_proxy_settings.cc
@@ -236,7 +236,7 @@
   }
 
   config->proxy_rules().bypass_rules.ParseFromStringUsingSuffixMatching(
-      JoinString(proxy_bypass_list_, ';'));
+      base::JoinString(proxy_bypass_list_, ";"));
 
   return true;
 }