tracing v2: Add base class for zero-copy protobuf (ProtoZero)

components/tracing/core/proto_utils.h

Index: components/tracing/core/proto_utils.h
diff --git a/components/tracing/core/proto_utils.h b/components/tracing/core/proto_utils.h
index 59b147db42faa88bc6acb71f57acd1b98cfce676..68599199688e98f0182aeb874a4e6e15e0585f2d 100644
--- a/components/tracing/core/proto_utils.h
+++ b/components/tracing/core/proto_utils.h
@@ -26,6 +26,10 @@ enum : uint32_t {
   kFieldTypeFixed32 = 5,
 };
 
+// Maximum message size supported: 256 MiB (4 x 7-bit due to varint encoding).
+constexpr size_t kMessageLengthFieldSize = 4;
+constexpr size_t kMaxMessageLength = (1u << (kMessageLengthFieldSize * 7)) - 1;
+
 // Returns the number of bytes sufficient to encode the largest
 // |int_size_in_bits|-bits integer using a non-redundant varint encoding.
 template <typename T>