Removed unnecessary WeakPtr returned by GetChromeObserver.

There was only one caller and the pointer was used immediately
so returning raw pointer instead.

Bug: 940840
Change-Id: I4e8efaa214370a7dbd4bb9096322dd58030d2a36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625274
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Chris Mumford <cmumford@google.com>
Cr-Commit-Position: refs/heads/master@{#662470}