WebSocket: Reduce dependencies on Document (2)

As a preparation for enabling to establish WebSocket connections off the main
thread, this CL reduces dependencies on Document in DocumentWebSocketChannel
for making it thread-safe. Specifically, this replaces GetDocument() calls for
the traces and the inspector with GetExecutionContext().

Bug: 825740
Change-Id: If2f4afd30c30453388de5e4e50ec7169025e8b4b
Reviewed-on: https://chromium-review.googlesource.com/984754
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547033}
6 files changed