Convert windows TaskQueue implementation to pimpl convention.

BUG=webrtc:8160,webrtc:8166

Review-Url: https://codereview.webrtc.org/3009133002
Cr-Original-Commit-Position: refs/heads/master@{#19709}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 341c8e40b7f7202b13a4b93e225f4ec8320c9eea