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Add a GetActiveTabUrl crosapi in LacrosChromeService to allow ash
to get the URL of active lacros tab. Show active lacros tab's
URL in feedback dialog when user press alt-shift-i from an active
lacros window.

TEST:
1. Launch lacros chrome, invoke feedback by alt-shift-i. Make sure
the URL field of the feedback dialog shows the url of active lacros
tab.
2. Launch lacros chrome, deactivate it by minimizing it, or activate
another app, invoke feedback by alt-shift-i. Make sure the URL field
of the feedback dialog is empty.
3. Launch both lacros chrome and ash chrome, minimize lacros chrome
or keep it in background, have ash chrome in the foreground, invoke
feedback by alt-shift-i. Make sure the URL field of the feedback
dialog shows the url of the active ash chrome tab.

Bug: 1142541
Change-Id: I3edc15db3fdda22214f3b849b24d4b619b219f59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505736
Commit-Queue: Jenny Zhang <jennyz@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Miriam Zimmerman <mutexlox@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822087}