Fix callback definition/usage in C++ bindings doc.

Mojo generated code now uses move-only base::OnceCallback
values, instead of const references to base::Callback, so
update the example code to reflect this.

Also add a small note explaining that a dependency needs to
be added from one of the top-level BUILD.gn groups to the new
interface GN target to ensure the compilation instructions
work correctly (otherwise, ninja complains that it doesn't
know about the target).

BUG=None
R=yzshen@chromium.org,sky@chromium.org

Change-Id: I2289af199da7da2bf8ed3ac0bd2f715d4cf8fbf0
Reviewed-on: https://chromium-review.googlesource.com/612387
Commit-Queue: David Turner <digit@chromium.org>
Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512161}

CrOS-Libchrome-Original-Commit: 743836f8512c249ab57749bcd2bcf9c696c991ea
1 file changed
tree: f3321e162227fee996c9acb33a32fb2a55bed6a5
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/