Remove distinction between lazy and non-lazy CallApiCallback, always explicitly set up target context

Bug: 
Change-Id: I0cb751a0415433fdfec21451e2fac3e0726bf26e
Reviewed-on: https://chromium-review.googlesource.com/743019
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49024}