Allow dynamic change to WebRTC's multiple_routes_enabled preference.
Allow dynamic change to WebRTC's multiple_routes_enabled preference.

Currently, this preference is not monitored such that any change through the extension api will require a restart of chrome.

landed fix for crbug.com/457629 is to expose extension api to allow this be changed dynamically.

Tested with the extension.

BUG=466197

Review URL: https://codereview.chromium.org/999833002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#320134}

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