Replace some asserts with DCHECKs

NOPRESUBMIT=true
BUG=webrtc:6779

Review-Url: https://codereview.webrtc.org/2535643002
Cr-Original-Commit-Position: refs/heads/master@{#15295}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: b890c95c33d2fceb8670625ae3e1783bb9bc9a44
13 files changed