Re-add AC3/EAC3 audio demuxing support

media/formats/mp4/es_descriptor.h

Index: media/formats/mp4/es_descriptor.h
diff --git a/media/formats/mp4/es_descriptor.h b/media/formats/mp4/es_descriptor.h
index 26ca86db2097f5ae5ff10d303b0acb63b4c9efc0..9066291457bb2c83af38f17076e961c6b65b60e7 100644
--- a/media/formats/mp4/es_descriptor.h
+++ b/media/formats/mp4/es_descriptor.h
@@ -21,8 +21,10 @@ namespace mp4 {
 // objectTypeIndication Values. Only values currently in use are included.
 enum ObjectType {
   kForbidden = 0,
-  kISO_14496_3 = 0x40,  // MPEG4 AAC
-  kISO_13818_7_AAC_LC = 0x67  // MPEG2 AAC-LC
+  kISO_14496_3 = 0x40,         // MPEG4 AAC

[ddorwin, 2016/01/04 23:22:17]

Are these complex names the best option? I wonder why this one doesn't contain
"AAC" like the next one.

[servolk, 2016/01/07 02:30:04]

I don't know. Git blame tells me these were added by acolwell@ and ycheo@ a
while ago. My only change in these lines is in adjusting the indentation of the
comments (and that was done automatically by  'git cl format'). I can change
these, if you insist, but in a separate CL, to keep this CL purely about
AC3/EAC3.

+  kISO_13818_7_AAC_LC = 0x67,  // MPEG2 AAC-LC
+  kAC3 = 0xa5,                 // AC3

[ddorwin, 2016/01/04 23:22:17]

Why do the names above have complex names while these have simple names? Is
there no ISO for them?

[servolk, 2016/01/07 02:30:04]

I believe the standard that describes AC3/EAC3 audio is ETSI TS 102 366, so we
could name those kETSI_TS_102_366_AC3 and kETSI_TS_102_366_EAC3, if you prefer.
Those shorted names were chosen by damienv@, I'm just upstreaming this from
Chromecast tree.

[ddorwin, 2016/01/07 19:12:43]

I think we should be consistent, though I'm not wild about the existing names.
dalecurtis@, WDYT?

+  kEAC3 = 0xa6                 // EAC3 / Dolby Digital Plus
 };
 
 // This class parse object type and decoder specific information from an