[tracing] Introduce "allowed_dump_modes" for memory dump config

base/trace_event/memory_dump_request_args.h

Index: base/trace_event/memory_dump_request_args.h
diff --git a/base/trace_event/memory_dump_request_args.h b/base/trace_event/memory_dump_request_args.h
index 74ec5dc18533c446e6fbd96672b20f0672de63a2..d96fdd4938bcccfcecaecb7804cddba5200a5ab5 100644
--- a/base/trace_event/memory_dump_request_args.h
+++ b/base/trace_event/memory_dump_request_args.h
@@ -28,7 +28,7 @@ enum class MemoryDumpType {
 };
 
 // Tells the MemoryDumpProvider(s) how much detailed their dumps should be.
-enum class MemoryDumpLevelOfDetail {
+enum class MemoryDumpLevelOfDetail : uint32_t {
   // For background tracing mode. The dump time is quick, and typically just the
   // totals are expected. Suballocations need not be specified. Dump name must
   // contain only pre-defined strings and string arguments cannot be added.