Remove const interfaces from content/public API.

Bug: 908139

Change-Id: I202059461f21fe87eee3d89ea8af45b8f0d009fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1595978
Reviewed-by: John Abd-El-Malek <[email protected]>
Commit-Queue: Lucas Gadani <[email protected]>
Cr-Commit-Position: refs/heads/master@{#657731}
82 files changed