Previously event timestamps were passed to VideoStream implementations
using OnInputEventReceived(). Now VideoStream gets a pointer to
InputEventTimestampSource, and then uses it to get event timestamp
when capturing each frame. This will allow to use fake input event
timestamps in performance tests.

Review-Url: https://codereview.chromium.org/2413553003
Cr-Commit-Position: refs/heads/master@{#425554}