Cleanup: Remove std::string("") usage and fix lint errors.

TBR=oshima@chromium.org,stevenjb@chromium.org,bajones@chromium.org,rch@chromium.org

Review URL: https://codereview.chromium.org/149573004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247844 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247844
Refine error code returned by ServiceWorkerVersion::StartWorker (e.g. PROCESS_NOT_FOUND etc)

BUG=313530

Review URL: https://codereview.chromium.org/147593006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247845 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247845