Avoid dangling pointer to Browser in AutofillContextMenuManager.

The context menu is, ultimately, owned by a WebContents:
- WebContentsImpl owns WebContentsView
- WebContentsView is implemented by WebContentsViewChildFrame, which
  owns WebContentsViewDelegate.
- WebContentsViewDelegate is implemented by
  ChromeWebContentsViewDelegateViews, which owns
  RenderViewContextMenuBase.
- RenderViewContextMenuBase is implemented by RenderViewContextMenu,
  which owns AutofillContextMenuManager.

Therefore the lifetime of the AutofillContextMenuManager is tied to that
of a WebContents. However, a WebContents can move from one Browser to
another and, in particular, a WebContents can outlive the Browser that
it started its life in if the Browser is closed after the move.
As a result, the raw pointer that AutofillContextMenuManager keeps to
its initial browser can become dangling.

This CL removes this raw pointer and obtains it dynamically as needed.

Fixed: 1493774
Change-Id: I5604ffd914fcdd2e738cdcf90b67cd4f1cc4cc51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5003544
Reviewed-by: Norge Vizcay <vizcay@google.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1220350}