Revert "Revert "Split EventWrapper in twain.""

This reverts commit cf3c83e76c273309558c86fda915410f65b7a899.

Reverting EventWrapper split did not fix the issue, re-landing.

BUG=chromium:470013
TBR=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49629004

Cr-Commit-Position: refs/heads/master@{#8946}
25 files changed