Clear the FrameBuffer in case of backward jumps in the picture id.

webrtc/modules/video_coding/frame_buffer2_unittest.cc

Index: webrtc/modules/video_coding/frame_buffer2_unittest.cc
diff --git a/webrtc/modules/video_coding/frame_buffer2_unittest.cc b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
index 6079bb95dd09cdad0033bce7d72b85faa70d7c9a..96be01f83de075fe21ddcb750843a3c1ce9c2734 100644
--- a/webrtc/modules/video_coding/frame_buffer2_unittest.cc
+++ b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
@@ -419,5 +419,22 @@ TEST_F(TestFrameBuffer2, LastContinuousFrameTwoLayers) {
   EXPECT_EQ(pid + 3, InsertFrame(pid + 3, 1, ts, true, pid + 2));
 }
 
+TEST_F(TestFrameBuffer2, PictureIdJumpBack) {
+  uint16_t pid = Rand();
+  uint32_t ts = Rand();
+
+  EXPECT_EQ(pid, InsertFrame(pid, 0, ts, false));
+  EXPECT_EQ(pid + 1, InsertFrame(pid + 1, 0, ts + 1, false, pid));
+  ExtractFrame();
+  CheckFrame(0, pid, 0);
+
+  // Jump back in pid but increase ts.
+  EXPECT_EQ(pid - 1, InsertFrame(pid - 1, 0, ts + 2, false));
+  ExtractFrame();
+  ExtractFrame();
+  CheckFrame(1, pid - 1, 0);
+  CheckNoFrame(2);
+}
+
 }  // namespace video_coding
 }  // namespace webrtc