This replaces external reference of Assistant logo in OOBE OptIn to
internal static icon.

Test: Manually
Bug: 828642
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I9d0c42acf53095af14c31fc1d64ab5f4731a9509
Reviewed-on: https://chromium-review.googlesource.com/994732Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yury Khmel <khmel@google.com>
Cr-Commit-Position: refs/heads/master@{#548103}