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- Adds UseCounters to count presentations started on secure/insecure
  origins.
- Logs a deprecation warning only when presentations are started,
  because of the warning frequency on PresentationRequest constructor or
  getAvailability.

Blink-dev thread:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/lumj0lVdtHA/discussion

Bug: 733381
Change-Id: I4352b2b9d3e887ade122bb8efb6e9a8efcbfd874
Reviewed-on: https://chromium-review.googlesource.com/578687
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488681}