[Gstreamer] Use gst_buffer_extract() in copyGstreamerBuffersToAudioChannel()
https://bugs.webkit.org/show_bug.cgi?id=113880

Reviewed by Philippe Normand.

copyGstreamerBuffersToAudioChannel() was mapping the GstBuffer content to memcpy
it to the AudioChannel buffer. This patch leverages gst_buffer_extract() to
simplify the code as it does exactly what we need: gst_buffer_map, memcpy,
gst_buffer_unmap and error handling.

Also replace GstBuffer NULL check by an assertion as we already make sure they
are not NULL before adding them to the list. Additionally, we now call
audioChannel->mutableData() only once instead of once per iteration. It is
a bit better as the getter does some work internally.

No new tests, no behavior change.

* platform/audio/gstreamer/AudioFileReaderGStreamer.cpp:
(WebCore::copyGstreamerBuffersToAudioChannel):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147567 268f45cc-cd09-0410-ab3c-d52691b4dbfc