Original CL: https://chromiumcodereview.appspot.com/2959263002/
Migrated to: https://chromium-review.googlesource.com/c/582927/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dabbb4dcaec76aa754f9576fcfcf1d0505564771

commit dabbb4dcaec76aa754f9576fcfcf1d0505564771
Author: Thanh Pham <thanhph@chromium.org>
Date: Tue Aug 08 19:41:22 2017

Show corresponding window,widget, and view in the UIElement tree when 
hovered.

Currently, in UI Devtools, when looking at the UI, users don't know 
which UIElement is associated with which DOM node so it's hard to see
or change its properties on the fly. This cl will identify the element
under mouse cursor when in inspection mode. Users can exit inspection
mode by left click or click on the blue box-in-arrow.

Bug:  736855 
Change-Id: Ie36b41701eb9744aad7c7914b2e197515c40ce3b
Reviewed-on: https://chromium-review.googlesource.com/582927
Commit-Queue: Thanh Pham <thanhph@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Terry Anderson <tdanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492729}
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/chrome/browser/ui/views/chrome_browser_main_extra_parts_views.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/BUILD.gn
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/protocol.json
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/BUILD.gn
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_devtools_css_agent.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_devtools_css_agent.h
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_devtools_dom_agent.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_devtools_dom_agent.h
[add] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_devtools_overlay_agent.cc
[add] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_devtools_overlay_agent.h
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_devtools_unittest.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_element.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/ui_element.h
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/view_element.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/view_element.h
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/widget_element.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/widget_element.h
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/window_element.cc
[modify] https://crrev.com/dabbb4dcaec76aa754f9576fcfcf1d0505564771/components/ui_devtools/views/window_element.h