Provides a better string (provides names of all implementations), but
also fixes a crash when accessing the ImplementationName() of
SimulcastEncoderAdapter where InitEncode has failed.
BUG=chromium:577932, webrtc:4897
R=stefan@webrtc.org
Review URL: https://codereview.webrtc.org/1599353003 .
Cr-Commit-Position: refs/heads/master@{#11321}