Introduce dchecked_cast, and start using it

It's the faster, less strict cousin of checked_cast.

BUG=none

Review-Url: https://codereview.webrtc.org/2714063002
Cr-Original-Commit-Position: refs/heads/master@{#16958}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d3edd770ade4a9323e136a39737957963cc676b7
11 files changed