Change pre_encode_callback to get a const frame.

Used only by tests. Deleted the EndToEndTest.UsesFrameCallbacks, which
modified pixel data. Change callback from in EndToEndTest.GetStats to call SleepMs, rath than
modifying the timestamp.

BUG=

Review URL: https://codereview.webrtc.org/1891733002

Cr-Original-Commit-Position: refs/heads/master@{#12406}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d30a1117f842f43e089404981361350b53b4f159
6 files changed