+ Correctly focus the inner WebContents when the attachment frame is
  focused.
+ Enable Enable WebViewFocusInteractiveTest.Focus_FocusBeforeNavigation.
+ Prevent BrowserPluginGuest from unfocusing the guest in oopif mode.

BUG=672947
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2715213009
Cr-Commit-Position: refs/heads/master@{#467296}