Sorry to hear that. Do you happen to know how third_party/devtools-frontend/src got deleted? If it is one of depot_tools scripts, or a command from the official documentation, please file a bug with more details so it can be fixed.
Regarding your current state, you have the following entry in .gclient custom_deps:
"src/third_party/devtools-frontend/src": None,
This tells gclient to ignore it, even if it's not cloned. I suggest you temporarily remove that line in .gclient file, run gclient sync, and then restore that line.