Enable UBSan float-cast-overflow warnings and fix existing ones

BUG=webrtc:8204

Review-Url: https://codereview.webrtc.org/3007153003
Cr-Original-Commit-Position: refs/heads/master@{#19694}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 30431d5acd6225dc66f4fe49498376377f4f87e9
3 files changed