Add histogram configs for new screenshare stats.
Add histogram configs for new screenshare stats.

Actual histograms are committed separately in the WebRTC repo.

BUG=554877

Review URL: https://codereview.chromium.org/1737083002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379259}

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