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This will allow consolidating many current "link-time" implementations
into virtual methods in our ContentClient classes, which should both
provide increased clarity and eliminate frequent internal build breaks
due to lag between a new public feature and matching internal CLs.

See discussion on https://codereview.chromium.org/1168643004/.

R=halliwell@chromium.org,lcwu@chromium.org,byungchul@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/1153173006

Cr-Commit-Position: refs/heads/master@{#333151}