Don't log an error on an empty manifest string

This happens in browser tests and this "error" is noise in the log.

BUG=339891

Review URL: https://codereview.chromium.org/136233006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249915 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249915
UMA server currently receives 2.5M unique thread name hashes in tracked objects. This huge number comes from generated thread names like "BrowserBlockingWorker1/23857". To reasonably reduce this number, this CL maps names like: "BrowserBlockingWorker1/23857" => "BrowserBlockingWorker1/*".
Same thing is done by chrome://profile page.

BUG=342004

Review URL: https://codereview.chromium.org/131453016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249916 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249916