RulesRegistry::Get(All)Rules now returns void

Currently, RulesRegistry::GetRules and GetAllRules return a std::string with an error message (with "" signalling no error). However, there is no execution path leading to returning anything else than "". Keeping the return type as std::string is confusing and makes unnecessary complicated. This CL changes the return type to void.

This CL is based off https://codereview.chromium.org/76013003/.

BUG=313241

Review URL: https://codereview.chromium.org/64093010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236222 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236222