Remove const interfaces from content/public API.

Bug: 908139
Tbr: sky@chromium.org, miu@chromium.org, boliu@chromium.org, seantopping@chromium.org
Change-Id: Ic005b1cc50020f130fe216efcc1c01187bcaa9fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1680425
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673393}
70 files changed