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The specification now allows a PushMessage to have valid non-empty data,
valid empty data, and null data. To support that, the content layer now uses
a PushEventPayload which can be null.

This also adds and updates relevant unit tests for each of the layers.

BUG=578036

Review URL: https://codereview.chromium.org/1636483002

Cr-Commit-Position: refs/heads/master@{#371839}