MSE: Test no muxed discontinuity forced by audio frame partial front trimming

This change adds frame processor tests to cover the behavior recently
landed from bad1c509 that changed "segments" appendMode's partial front
trimming logic.

BUG=408216
R=dalecurtis@chromium.org
TEST=new segments test fails before and passes after bad1c509, new sequence test passes before and after

Review URL: https://codereview.chromium.org/567403004

Cr-Commit-Position: refs/heads/master@{#294920}

media/filters/frame_processor_unittest.cc

@@ -664,6 +664,30 @@ TEST_P(FrameProcessorTest, PartialAppendWindowFilterNoDiscontinuity) {
   CheckReadsThenReadStalls(audio_.get(), "7:0 19");
 }
 
+TEST_P(FrameProcessorTest, PartialAppendWindowFilterNoNewMediaSegment) {
+  // Tests that a new media segment is not forcibly signalled for audio frame
+  // partial front trim, to prevent incorrect introduction of a discontinuity
+  // and potentially a non-keyframe video frame to be processed next after the
+  // discontinuity.
+  InSequence s;
+  AddTestTracks(HAS_AUDIO | HAS_VIDEO);
+  new_media_segment_ = true;
+  frame_processor_->SetSequenceMode(GetParam());
+  EXPECT_CALL(callbacks_, PossibleDurationIncrease(frame_duration_));
+  ProcessFrames("", "0K");
+  EXPECT_CALL(callbacks_, PossibleDurationIncrease(frame_duration_));
+  ProcessFrames("-5K", "");
+  EXPECT_CALL(callbacks_, PossibleDurationIncrease(frame_duration_* 2));
+  ProcessFrames("", "10");
+
+  EXPECT_EQ(base::TimeDelta(), timestamp_offset_);
+  EXPECT_FALSE(new_media_segment_);
+  CheckExpectedRangesByTimestamp(audio_.get(), "{ [0,5) }");
+  CheckExpectedRangesByTimestamp(video_.get(), "{ [0,20) }");
+  CheckReadsThenReadStalls(audio_.get(), "0:-5");
+  CheckReadsThenReadStalls(video_.get(), "0 10");
+}
+
 INSTANTIATE_TEST_CASE_P(SequenceMode, FrameProcessorTest, Values(true));
 INSTANTIATE_TEST_CASE_P(SegmentsMode, FrameProcessorTest, Values(false));