RemoteMacViews: Fix WebContentsView::Focus

Make WebContentsView::Focus use RenderWidgetHostViewBase::Focus, which
will get forwarded to the app shim process appropriately.

Change-Id: I47e1404c8fc918427bc6b739d2d9d8a4edd946d3
Reviewed-on: https://chromium-review.googlesource.com/c/1408809
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622760}
2 files changed