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This CL corrects the timestamp sent with the encoded frame data from
VTVideoEncodeAccelerator. Recently, this functionality is added to the
VEA interface and we can use the frame's original capture timestamp.

BUG=350106
TEST=AppRTC loopback in Macbook Air.

Review-Url: https://codereview.chromium.org/2039423002
Cr-Commit-Position: refs/heads/master@{#398187}