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In order to make the code changes easier to reason about, this is now part 1
of a two-part commit. In this commit, the existing code is moved from
content/common and content/child into content/renderer (it isn't used anywhere
else). In part 2, the keys are moved out of content into the embedder.

Part 2: https://codereview.chromium.org/1653263005/

BUG=543220

Review URL: https://codereview.chromium.org/1680873002

Cr-Commit-Position: refs/heads/master@{#374763}