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SnapshotController implementation. It will be used in WebContentsObservers for Offline Pages - to detect the right moment to make a snapshot.
It will take input and generate a sequence of StartSnapshot calls (typically 1 or 2) to its Client.
This is initial implementation, in the future we might update the logic to rely not on a timer and onload event,
but rather on some more specific indicators (meaningful layout or proportion of resources loaded).

BUG=606106

Review URL: https://codereview.chromium.org/1920603002

Cr-Commit-Position: refs/heads/master@{#389843}