Browser Plugin: Make aware of focus direction.

In order to fix focus ordering into and out of GuestViews, this CL makes focus
aware of tab direction.

This CL is the content side change for this Blink change:
https://codereview.chromium.org/852083002

BUG=448716, 147644
TBR=kenrb@chromium.org for trivial browser_plugin_messages.h change (added focus type to existing IPC).

Review URL: https://codereview.chromium.org/853333002

Cr-Commit-Position: refs/heads/master@{#312126}
17 files changed