GitLab

The attribution methods are only available in R, so add a ApiHelperForR
class to access them.

Bug: 1085041
Change-Id: Ief6957ad59cd499111c96a7a171eb2dca8e3d609
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570034Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833335}