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This CL removes the use of TabModel in UserActivityHandler which was used
for getting WebStateList; and uses a browser to get WebStateList from
BrowserInterfaceProvider.

Change-Id: I2422071eccd7a5429b195e1b21be551f3bf48fce
Bug: 1049845, 1045575
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153027Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Nazerke Kalidolda <nazerke@google.com>
Cr-Commit-Position: refs/heads/master@{#762788}