SPM: Fix EmbedRoot focus

- Notify FocusSynchronizer when a window in EmbedRoot is focused
  so that WS side would focused its proxy window;
- Observe FocusSynchronizer to reset the focused window in EmbedRoot
  when the FocusSynchronizer changes to different focus client so
  that the observers of the focused window gets the expected "blur"

Bug: b/127864880
Change-Id: I00f76dd36040b3fdff05a8130093596aa5be6ffa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525057
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641213}
4 files changed