Improves comments in media::ChannelLayoutToChannelCount

It is now more clear that this method can in fact return 0 for some
unique input parameters.

Tbr: olka
Bug: 1052825
Change-Id: Ib7aaa66ea2f37b45fffdde89a1835ccc511d5edb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2060294Reviewed-by: Henrik Andreasson <henrika@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Henrik Andreasson <henrika@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742131}

media/base/channel_layout.h

@@ -143,7 +143,12 @@ constexpr int kMaxConcurrentChannels = 8;
 // from 0 to ChannelLayoutToChannelCount(layout) - 1.
 MEDIA_SHMEM_EXPORT int ChannelOrder(ChannelLayout layout, Channels channel);
 
-// Returns the number of channels in a given ChannelLayout.
+// Returns the number of channels in a given ChannelLayout or 0 if the
+// channel layout can't be mapped to a valid value. Currently, 0
+// is returned for CHANNEL_LAYOUT_NONE, CHANNEL_LAYOUT_UNSUPPORTED,
+// CHANNEL_LAYOUT_DISCRETE, and CHANNEL_LAYOUT_BITSTREAM. For these cases,
+// additional steps must be taken to manually figure out the corresponding
+// number of channels.
 MEDIA_SHMEM_EXPORT int ChannelLayoutToChannelCount(ChannelLayout layout);
 
 // Given the number of channels, return the best layout,