Add AssistContent class to third_party

BUG=497258

Add assistcontent class to third party to pass URI.

Change-Id: Ie648fc9de2bd41e26e281bde8f39c101bfde9822
Reviewed-on: https://chromium-review.googlesource.com/288803
Reviewed-by: Aurimas Liutikas <aurimas@chromium.org>
Commit-Queue: Aurimas Liutikas <aurimas@chromium.org>
Tested-by: Aurimas Liutikas <aurimas@chromium.org>