Catching more errors when parsing ICE server URLs.

Every malformed URL should now produce an error message in JS, rather than
silently failing and possibly printing a warning message to the console (and
possibly crashing).

Also added some unit tests, and made "ParseIceServers" public.

BUG=445002

Review URL: https://codereview.webrtc.org/1344143002

Cr-Commit-Position: refs/heads/master@{#10186}

webrtc/base/stringencode.h

@@ -146,6 +146,11 @@ size_t split(const std::string& source, char delimiter,
 size_t tokenize(const std::string& source, char delimiter,
                 std::vector<std::string>* fields);
 
+// Tokenize, including the empty tokens.
+size_t tokenize_with_empty_tokens(const std::string& source,
+                                  char delimiter,
+                                  std::vector<std::string>* fields);
+
 // Tokenize and append the tokens to fields. Return the new size of fields.
 size_t tokenize_append(const std::string& source, char delimiter,
                        std::vector<std::string>* fields);