WebSocket: Remove unused WorkerWebSocketChannel

WebSockets on workers now run off the main thread. This CL removes unused
on-the-main-thread code for cleanup.

Bug: 825740
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I5cf5b846e6099067d2da9b8b90de157e09020d7c
Reviewed-on: https://chromium-review.googlesource.com/1146134
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577146}
32 files changed