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All subclasses must implement the method, even if it should be empty.
This would ensure the compiler makes a developer aware of the lifecycle
concerns around RenderFrame/RenderView, and avoid the unintended
transfering ownership when inheriting the observer interface.

BUG=616214

Review-Url: https://codereview.chromium.org/2034153002
Cr-Commit-Position: refs/heads/master@{#398395}