These functions operate directly on the packet masks, and are thus not directly
involved in the FEC encoding/decoding operation. The ::internal namespace is used
for packet mask-related functions, and will be renamed later on.
This CL should have no impact on functionality or performance.
BUG=webrtc:5654
Review-Url: https://codereview.webrtc.org/2269893002
Cr-Commit-Position: refs/heads/master@{#13914}