Fix the scrim view not showing for DevTools dialogs

DevTools switched from using infobars to using proper modal
dialogs for prompts. This works except for the case when DevTools
is docked and, thus, not found in the TabStrip. This CL addresses
the issue by adding a DevTools scrim view similar to the content
scrim view to cover the DevTools view if DevTools's WebContents
gets a modal dialog shown. In the implementation, it happens
when a WebContents is not found in the TabStrip model and it is
gated upon an additional check verifying that WebContents does
have a DevTools window.

Screencast: http://go/scrcast/NjI0NjEzODczMzY1ODExMnwxNWZkZTIxNS0zZQ
Fixed: 406218346
Change-Id: Ie304750412ddc17a4e64e33f6554c026d0a98aa4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6480103
Reviewed-by: Jan Keitel <jkeitel@google.com>
Reviewed-by: Wolfgang Beyer <wolfi@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1451032}