Exclude devtools windows from incognito windows count.

|GetIncognitoSessionsActiveForProfile| function is updated not to
count DevTools windows as incognito windows (e.g. when they are opened
on incognito windows and detached).
Browser tests are updated to cover this change.

Bug: 936328, 896235
Change-Id: Id68a5fec4cf60471e7a9d54c6393f0eabf541e7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1495526
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637692}
3 files changed