Use accessor methods for EncodedImage timestamp

Preparation for landing webrtc cl
https://webrtc-review.googlesource.com/c/src/+/95103.

Bug: webrtc:9378
Change-Id: Ia2510f51392ee7ce7a4c12a791a0efc120f06524
Reviewed-on: https://chromium-review.googlesource.com/1219586
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590389}
6 files changed