MSE: Don't force discontinuity logic on audio frame partial front trimming

This change removes the signalling of a new media segment when an audio
frame is partially front-trimmed. This allows other muxed streams, like
video, to not necessarily continue with a keyframe as the next frame
processed (unless new media segment has otherwise been signalled).

BUG=408216
R=dalecurtis@chromium.org
TEST=no media_unittest or mediasource layout test regression

Review URL: https://codereview.chromium.org/559843005

Cr-Commit-Position: refs/heads/master@{#294885}

media/filters/frame_processor.cc

@@ -598,12 +598,6 @@ bool FrameProcessor::ProcessFrame(
         HandlePartialAppendWindowTrimming(append_window_start,
                                           append_window_end,
                                           frame)) {
-      // If |frame| was front-trimmed a discontinuity may exist, so treat the
-      // next frames appended as if they were the beginning of a new media
-      // segment.
-      if (frame->timestamp() != presentation_timestamp && !sequence_mode_)
-        *new_media_segment = true;
-
       // |frame| has been partially trimmed or had preroll added.  Though
       // |frame|'s duration may have changed, do not update |frame_duration|
       // here, so |track_buffer|'s last frame duration update uses original