Issue: This API was calculating the file_header and frame_header offset only for the first frame which is not the right logic. We need to skip the file and frame header every time we extract a new frame.
Also added a unit test case which compares the extracted frame with the frame stored in text file.
BUG=webrtc:6761
NOPRESUBMIT=true
NOTRY=true
Review-Url: https://codereview.webrtc.org/2529923002
Cr-Commit-Position: refs/heads/master@{#15260}
b7636b4656