GitLab

Cherry-picks a couple of things:

  - Adds support for wrapping and unwrapping new base shm regions
  - Ports one use (PDF compositor test code) to use the above
  - Updates base::SharedMemory wrapping to force read-only regions
    when wrapping a handle as read-only

Bug: 826213
Change-Id: I11d923b23688454ffdbf26ba5efa93ba1605b855
Reviewed-on: https://chromium-review.googlesource.com/992360Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547728}