Support removing b=AS bandwidth constraints.

In code this is represented as setting -1 to max_bandwidth_bps, but this
value was being ignored by webrtcvideoengine2.cc, so previous restrictions
would still apply.

BUG=webrtc:6202
TEST=Setting "unlimited" for Bandwidth in Chromium in https://webrtc.github.io/samples/src/content/peerconnection/bandwidth/.
R=deadbeef@webrtc.org,stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/2740783006
Cr-Original-Commit-Position: refs/heads/master@{#17175}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5c7760a3a2a72b905705e003f62b60eabfda7ff2