Add initialization and per frame flag members

Modified EncoderTest class to have separate member variables
for initialization time and per-frame.

Change-Id: I08a1901f8f3ec16e45f96297e08e7f6df0f4aa0b
diff --git a/test/keyframe_test.cc b/test/keyframe_test.cc
index 19c7152..b003aa3 100644
--- a/test/keyframe_test.cc
+++ b/test/keyframe_test.cc
@@ -34,7 +34,7 @@
   virtual void PreEncodeFrameHook(::libvpx_test::VideoSource *video,
                                   ::libvpx_test::Encoder *encoder) {
     if (kf_do_force_kf_)
-      flags_ = (video->frame() % 3) ? 0 : VPX_EFLAG_FORCE_KF;
+      frame_flags_ = (video->frame() % 3) ? 0 : VPX_EFLAG_FORCE_KF;
     if (set_cpu_used_ && video->frame() == 1)
       encoder->Control(VP8E_SET_CPUUSED, set_cpu_used_);
   }