[DevTools] Add more descriptive UI for state transitions.

This CL incorporates some of the changes discussed in the latest UX meeting:
- Reorder the columns and toolbar buttons
- Display the metadata in a manner to a similar to the Network panel
- Add instructions for the inital landing page and other UI states

Bug: 942174
Change-Id: I7bd451377b241ec70d49c6d0b443522b43e5bcfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578554
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653543}
3 files changed