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}