peerconnection: prefer spec names for signaling state
peerconnection: prefer spec names for signaling state

Map the internal state names to the spec ones defined in
  https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum
instead of exposing them. This only affects the (not specified)
error strings.

Bug: None
Change-Id: Ib0b35bb3106b1688e8386f6fdd0b8c7fdebaf1dc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178390
Reviewed-by: Henrik Boström <106ab6647ddadbf38d6259e9d5ec4de77fa52c00@webrtc.org>
Commit-Queue: Philipp Hancke <563b103f57dc2044b12822c2a7559aa15cb1c987@googlemail.com>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#31591}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc