Revert "Remove ChromeActivity reference from AutoCompleteMediator"

This reverts commit 8315bbca.

Reason for revert: Doesn't support multi-window

Original change's description:
> Remove ChromeActivity reference from AutoCompleteMediator
> 
> ChromeActivity is used in AutocompleteMediator to provide the current
> TabModelSelector. This dependency should instead be acquired through
> direct constructor injection.
> 
> Bug: 1123209,1097292,1113364
> Change-Id: I95c8a58f06f250b6397ec7926b2262b5b2b4298b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382958


> Reviewed-by: Patrick Noland <pnoland@chromium.org>
> Reviewed-by: Ted Choc <tedchoc@chromium.org>
> Reviewed-by: Tomasz Wiszkowski <ender@google.com>
> Commit-Queue: Brandon Wylie <wylieb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#804419}

TBR=tedchoc@chromium.org,wylieb@chromium.org,pnoland@chromium.org,ender@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1123209
Bug: 1097292
Bug: 1113364
Change-Id: Ib990eba5cc00f7b5209c90b5371385dfe5af9283
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402087


Reviewed-by: Brandon Wylie <wylieb@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805543}