gn: bootstrap: add base/task_scheduler/task_tracker_posix.cc to sources

Review-Url: https://codereview.chromium.org/2480253005
Cr-Original-Commit-Position: refs/heads/master@{#430670}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: aaeeef1bb01f6ff4154652e0999ae4a8834324ca