Add support for avc3 codec string.

media/mp4/box_definitions.cc

Index: media/mp4/box_definitions.cc
diff --git a/media/mp4/box_definitions.cc b/media/mp4/box_definitions.cc
index e7f169323bb6b90a94854d9bfe9ce55bc07bcd3e..218e5bb09205c8c46b35d92631a4e199c1671e53 100644
--- a/media/mp4/box_definitions.cc
+++ b/media/mp4/box_definitions.cc
@@ -399,13 +399,18 @@ bool VideoSampleEntry::Parse(BoxReader* reader) {
     }
   }
 
-  if (format == FOURCC_AVC1 ||
-      (format == FOURCC_ENCV && sinf.format.format == FOURCC_AVC1)) {
+  if (IsFormatValid())
     RCHECK(reader->ReadChild(&avcc));
-  }
+
   return true;
 }
 
+bool VideoSampleEntry::IsFormatValid() const {
+  return format == FOURCC_AVC1 || format == FOURCC_AVC3 ||
+      (format == FOURCC_ENCV && (sinf.format.format == FOURCC_AVC1 ||
+                                 sinf.format.format == FOURCC_AVC3));
+}
+
 ElementaryStreamDescriptor::ElementaryStreamDescriptor()
     : object_type(kForbidden) {}