DevTools: Move devtools_agent.mojom from public/web/ to public/mojom/devtools

This CL just moves devtools_agent.mojom from public/web/ to
public/mojom/devtools/ for cleanup.

This is useful for onion-soupifying shared workers without an additional
dependency in BUILD.gn.

Bug: 911415
Change-Id: I19aaf2fb8283b69fa44c4573b30a2cfe5955b94f
Reviewed-on: https://chromium-review.googlesource.com/c/1368084
Reviewed-by: Dmitry Gozman <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Commit-Queue: Hiroki Nakagawa <[email protected]>
Cr-Commit-Position: refs/heads/master@{#615043}
25 files changed