Report errors in RTCPeerConnection legacy functions via the the failure callback instead of exceptions.

This makes createAnswer() and createOffer() more compliant with the spec and will reduce unexpected surprises when their return value changes to Promise<void>.

BUG=499126

Review URL: https://codereview.chromium.org/1713953002

Cr-Commit-Position: refs/heads/master@{#380614}
9 files changed