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

media/base/audio_buffer.cc

Index: media/base/audio_buffer.cc
diff --git a/media/base/audio_buffer.cc b/media/base/audio_buffer.cc
index 4b972b90a1abda47c363b0cd46c7c8362ca60ee2..61d73a322a41d71ddf9453851ec2b148f0ebe662 100644
--- a/media/base/audio_buffer.cc
+++ b/media/base/audio_buffer.cc
@@ -275,4 +275,40 @@ void AudioBuffer::TrimEnd(int frames_to_trim) {
   adjusted_frame_count_ -= frames_to_trim;
 }
 
+void AudioBuffer::TrimRange(int start, int end) {
+  const int frames_to_trim = end - start;
+  CHECK_GE(frames_to_trim, 0);
+  CHECK_LE(frames_to_trim, adjusted_frame_count_);
+
+  const int bytes_per_channel = SampleFormatToBytesPerChannel(sample_format_);
+  const int frames_to_copy = adjusted_frame_count_ - frames_to_trim;
+  switch (sample_format_) {
+    case kSampleFormatPlanarS16:
+    case kSampleFormatPlanarF32:
+      // Planar data must be shifted per channel.
+      for (int ch = 0; ch < channel_count_; ++ch) {
+        memcpy(channel_data_[ch] + start * bytes_per_channel,
+               channel_data_[ch] + end * bytes_per_channel,
+               bytes_per_channel * frames_to_copy);
+      }
+      break;
+    case kSampleFormatU8:
+    case kSampleFormatS16:
+    case kSampleFormatS32:
+    case kSampleFormatF32: {
+      // Interleaved data can be shifted all at once.
+      const int frame_size = channel_count_ * bytes_per_channel;
+      memcpy(channel_data_[0] + start * frame_size,
+             channel_data_[0] + end * frame_size,
+             frame_size * frames_to_copy);
+      break;
+    }
+    case kUnknownSampleFormat:
+      NOTREACHED() << "Invalid sample format!";
+  }
+
+  // Trim the leftover data off the end of the buffer and update duration.
+  TrimEnd(frames_to_trim);
+}
+
 }  // namespace media