Convert mac/ios TaskQueue implementation to pimpl convention.

BUG=webrtc:8160, webrtc:8166

Review-Url: https://codereview.webrtc.org/3013433002
Cr-Original-Commit-Position: refs/heads/master@{#19743}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 66d07c54fef7b0ddd5418f05096757e2acc8308e