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This separates the Chromium side of requestMediaKeySystemAccess() into the Blink interface implementation (WebEncryptedMediaClientImpl), which also handles the UMAs, and the core algorithm (KeySystemConfigSelector). This should make it easy to test the core algorithm.

BUG=468912

Review URL: https://codereview.chromium.org/1052273004

Cr-Commit-Position: refs/heads/master@{#323983}