Support start trimming post-decoding. Use it with FFmpegDemuxer.

media/base/audio_buffer_unittest.cc

Index: media/base/audio_buffer_unittest.cc
diff --git a/media/base/audio_buffer_unittest.cc b/media/base/audio_buffer_unittest.cc
index c40c076bd5e8ee73c224df4db6b972f0c00fea84..09da83e5fa45973c2ab8da63f6c60b2ee31c3068 100644
--- a/media/base/audio_buffer_unittest.cc
+++ b/media/base/audio_buffer_unittest.cc
@@ -358,4 +358,49 @@ TEST(AudioBufferTest, Trim) {
   EXPECT_EQ(buffer->duration(), base::TimeDelta::FromSeconds(0));
 }
 
+TEST(AudioBufferTest, TrimRange) {
+  const ChannelLayout channel_layout = CHANNEL_LAYOUT_4_0;
+  const int channels = ChannelLayoutToChannelCount(channel_layout);
+  const int frames = 100;
+  const base::TimeDelta start_time;
+  const base::TimeDelta duration = base::TimeDelta::FromSeconds(frames);
+  const float step = 1.0f;
+  scoped_refptr<AudioBuffer> buffer =
+      MakeAudioBuffer<float>(kSampleFormatPlanarF32,
+                             channel_layout,
+                             channels,
+                             kSampleRate,
+                             0,
+                             step,
+                             frames,
+                             start_time,
+                             duration);
+  EXPECT_EQ(frames, buffer->frame_count());
+  EXPECT_EQ(start_time, buffer->timestamp());
+  EXPECT_EQ(frames, buffer->duration().InSeconds());
+
+  scoped_ptr<AudioBus> bus = AudioBus::Create(channels, frames);
+
+  const int trim_start = 25;
+  const int trim_length = 50;
+
+  // Verify the first 25 frames before trimming.
+  buffer->ReadFrames(trim_start, 0, 0, bus.get());
+  VerifyResult(bus->channel(0), trim_start, 0, step);
+
+  // Trim frames from the middle of the buffer.
+  buffer->TrimRange(trim_start, trim_start + trim_length);
+  EXPECT_EQ(buffer->frame_count(), frames - trim_length);
+  EXPECT_EQ(buffer->timestamp(), start_time);
+  EXPECT_EQ(buffer->duration(), base::TimeDelta::FromSeconds(50));
+
+  // Verify the first 25 frames.
+  buffer->ReadFrames(trim_start, 0, 0, bus.get());
+  VerifyResult(bus->channel(0), trim_start, 0, step);
+
+  // Verify the next 25 frames.
+  buffer->ReadFrames(trim_start, trim_start, 0, bus.get());
+  VerifyResult(bus->channel(0), trim_start, trim_start + trim_length, step);
+}
+
 }  // namespace media