Since ESC has been used on UI frontend for toggle Drawer
https://developers.google.com/web/tools/chrome-devtools/ui#drawer-tabs, we will not rely on ESC, instead of reuse the Ctrl/Cmd+Shift+C shortcut for toggle inspector mode. Clicking ElementInspect button should still work though.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/94735ad643124ce67f3acffc6b816dac9323129e

commit 94735ad643124ce67f3acffc6b816dac9323129e
Author: Wei Li <weili@chromium.org>
Date: Wed Oct 31 17:37:16 2018

UIDevTools: Make exiting inspector mode work

Clicking ElementInspect button would toggle entering/exiting
inspector mode. However, previously clicking that button in
inspector mode would be handled by pinned element logic. In this
CL, we capture returned window element's active state. If the
window is not active, pinned element logic should not mark the
event as handled to give frontend a chance to handle the event.

BUG= 899000 

Change-Id: I4356f85f0ec6d90298473bb894f197aa4ab733e3
Reviewed-on: https://chromium-review.googlesource.com/c/1306403
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604317}
[modify] https://crrev.com/94735ad643124ce67f3acffc6b816dac9323129e/components/ui_devtools/views/overlay_agent_aura.cc