Clear the FrameBuffer in case of a backward jump in the picture id.

Even though this is against the spec we allow a stream to continue if
a backwards jump in the picture id occurs on a keyframe.

BUG=webrtc:7001, webrtc:5514

Review-Url: https://codereview.webrtc.org/2640793003
Cr-Commit-Position: refs/heads/master@{#16146}
diff --git a/webrtc/modules/video_coding/frame_buffer2_unittest.cc b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
index 6079bb9..96be01f 100644
--- a/webrtc/modules/video_coding/frame_buffer2_unittest.cc
+++ b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
@@ -419,5 +419,22 @@
   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