compositor proxy: Document the serialization tag.

Source/bindings/core/v8/SerializationTag.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SerializationTag_h
 #define SerializationTag_h
 
 namespace blink {
 
 // Serialization format is a sequence of tags followed by zero or more data arguments.
@@ skipping 60 matching lines @@
         //                     props = algorithmId:uint32_t, type:uint32_t, namedCurve:uint32_t
     ObjectReferenceTag = '^', // ref:uint32_t -> reference table[ref]
     GenerateFreshObjectTag = 'o', // -> empty object allocated an object ID and pushed onto the open stack (ref)
     GenerateFreshSparseArrayTag = 'a', // length:uint32_t -> empty array[length] allocated an object ID and pushed onto the open stack (ref)
     GenerateFreshDenseArrayTag = 'A', // length:uint32_t -> empty array[length] allocated an object ID and pushed onto the open stack (ref)
     ReferenceCountTag = '?', // refTableSize:uint32_t -> If the reference table is not refTableSize big, fails.
     StringObjectTag = 's', //  string:RawString -> new String(string) (ref)
     NumberObjectTag = 'n', // value:double -> new Number(value) (ref)
     TrueObjectTag = 'y', // new Boolean(true) (ref)
     FalseObjectTag = 'x', // new Boolean(false) (ref)
-    CompositorProxyTag = 'C',
+    CompositorProxyTag = 'C', // elementId:uint64_t, bitfields:uint32_t -> CompositorProxy (ref)
     VersionTag = 0xFF // version:uint32_t -> Uses this as the file version.
 };
 
 enum ArrayBufferViewSubTag {
     ByteArrayTag = 'b',
     UnsignedByteArrayTag = 'B',
     UnsignedByteClampedArrayTag = 'C',
     ShortArrayTag = 'w',
     UnsignedShortArrayTag = 'W',
     IntArrayTag = 'd',
     UnsignedIntArrayTag = 'D',
     FloatArrayTag = 'f',
     DoubleArrayTag = 'F',
     DataViewTag = '?'
 };
 
 } // namespace blink
 
 #endif