Add channel_count parameter back to AudioBuffer creation methods.

This fixes a bug where we failed to set the proper channel count on
AudioBuffers with the CHANNEL_LAYOUT_DISCRETE channel layout.

BUG=357801
R=dalecurtis@chromium.org
TBR=dmichael

Review URL: https://codereview.chromium.org/212103013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260333 0039d316-1c4b-4281-b951-d872f2087c98

• content/renderer/pepper/content_decryptor_delegate.cc[diff]
• media/base/audio_buffer.cc[diff]
• media/base/audio_buffer.h[diff]
• media/base/audio_buffer_converter.cc[diff]
• media/base/audio_buffer_converter_unittest.cc[diff]
• media/base/audio_buffer_queue_unittest.cc[diff]
• media/base/audio_buffer_unittest.cc[diff]
• media/base/audio_splicer.cc[diff]
• media/base/audio_splicer_unittest.cc[diff]
• media/base/test_helpers.cc[diff]
• media/base/test_helpers.h[diff]
• media/filters/audio_renderer_algorithm_unittest.cc[diff]
• media/filters/audio_renderer_impl_unittest.cc[diff]
• media/filters/decrypting_audio_decoder_unittest.cc[diff]
• media/filters/ffmpeg_audio_decoder.cc[diff]
• media/filters/opus_audio_decoder.cc[diff]