Remove content flag for Accessibility Object Model

Use the blink-enabled-features command-line flag to enable it
instead, it's simpler.

BUG=680345

Review-Url: https://codereview.chromium.org/2859333002
Cr-Commit-Position: refs/heads/master@{#469737}
7 files changed