[PH2][Refactor][Minor] Remove magic number for clarity (grpc#38166)

[PH2][Refactor][Minor] Remove magic number for clarity.
Using GRPC_HEADER_SIZE_IN_BYTES to make clear that we are parsing the gRPC header.

Closes grpc#38166

COPYBARA_INTEGRATE_REVIEW=grpc#38166 from tanvi-jagtap:remove_magic_number f35baa3
PiperOrigin-RevId: 700536771

src/core/ext/transport/chttp2/transport/frame_data.cc

@@ -84,13 +84,16 @@ grpc_core::Poll<grpc_error_handle> grpc_deframe_unprocessed_incoming_frames(
   grpc_slice_buffer* slices = &s->frame_storage;
   grpc_error_handle error;
 
-  if (slices->length < 5) {
-    if (min_progress_size != nullptr) *min_progress_size = 5 - slices->length;
+  if (slices->length < GRPC_HEADER_SIZE_IN_BYTES) {
+    if (min_progress_size != nullptr) {
+      *min_progress_size = GRPC_HEADER_SIZE_IN_BYTES - slices->length;
+    }
     return grpc_core::Pending{};
   }
 
-  uint8_t header[5];
-  grpc_slice_buffer_copy_first_into_buffer(slices, 5, header);
+  uint8_t header[GRPC_HEADER_SIZE_IN_BYTES];
+  grpc_slice_buffer_copy_first_into_buffer(slices, GRPC_HEADER_SIZE_IN_BYTES,
+                                           header);
 
   switch (header[0]) {
     case 0:
@@ -114,18 +117,20 @@ grpc_core::Poll<grpc_error_handle> grpc_deframe_unprocessed_incoming_frames(
                   (static_cast<uint32_t>(header[3]) << 8) |
                   static_cast<uint32_t>(header[4]);
 
-  if (slices->length < length + 5) {
+  if (slices->length < length + GRPC_HEADER_SIZE_IN_BYTES) {
     if (min_progress_size != nullptr) {
-      *min_progress_size = length + 5 - slices->length;
+      *min_progress_size = length + GRPC_HEADER_SIZE_IN_BYTES - slices->length;
     }
     return grpc_core::Pending{};
   }
 
   if (min_progress_size != nullptr) *min_progress_size = 0;
 
   if (stream_out != nullptr) {
-    s->call_tracer_wrapper.RecordIncomingBytes({5, length, 0});
-    grpc_slice_buffer_move_first_into_buffer(slices, 5, header);
+    s->call_tracer_wrapper.RecordIncomingBytes(
+        {GRPC_HEADER_SIZE_IN_BYTES, length, 0});
+    grpc_slice_buffer_move_first_into_buffer(slices, GRPC_HEADER_SIZE_IN_BYTES,
+                                             header);
     grpc_slice_buffer_move_first(slices, length, stream_out->c_slice_buffer());
   }