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}

webrtc/modules/video_coding/main/source/receiver_unittest.cc

@@ -31,8 +31,8 @@ class TestVCMReceiver : public ::testing::Test {
       : clock_(new SimulatedClock(0)),
         timing_(clock_.get()),
         receiver_(&timing_, clock_.get(), &event_factory_) {
-    stream_generator_.reset(new
-        StreamGenerator(0, 0, clock_->TimeInMilliseconds()));
+    stream_generator_.reset(
+        new StreamGenerator(0, clock_->TimeInMilliseconds()));
   }
 
   virtual void SetUp() {