Remove the timestamp_ member of StreamGenerator.
timestamp_ is only used in GenerateFrame() and its old value is
discarded. So it just needs to be a local variable in GenerateFrame().
As a result, we can remove the start_timestamp parameter from the
constructor and Init().
Also mark the GeneratePacket() method private because it is only used
internally.
R=stefan@webrtc.org
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/50149004
Cr-Commit-Position: refs/heads/master@{#9386}
4 files changed
