Remove some unused includes in headless/

Bug: 
Change-Id: Icb5351bb6112fc89e36dab82c15f32887dab9217
Reviewed-on: https://chromium-review.googlesource.com/720594
Reviewed-by: David Vallet <dvallet@chromium.org>
Commit-Queue: Iris Uy <irisu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509313}