DevTools: Code Folding

This adds the ability to collapse sections of code in the Sources and
Network panels.

https://i.imgur.com/CkREJEV.png

Bug: 328431

Change-Id: I88e44d86c7fd2e6dbf076cc734aa538836516cf0
Reviewed-on: https://chromium-review.googlesource.com/c/1014386
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616885}
12 files changed