[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..0edac9a59a4b186e88e90cafeaa8471abdf8e64d 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 {

[oystein (OOO til 10th of July), 2016/06/20 17:02:18]

Why do you need to explicitly specify the storage type?

[ssid, 2016/06/20 19:12:06]

Because I am casting this later, so thought it is safer to specify the type here
as well.

   // 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.
@@ -44,7 +44,7 @@ enum class MemoryDumpLevelOfDetail {
   // Unrestricted amount of entries per dump.
   DETAILED,
 
-  // For IPC Macros.
+  FIRST = BACKGROUND,

[oystein (OOO til 10th of July), 2016/06/20 17:02:18]

Nit: "FIRST" should probably be first in the list, to make it more obvious that
it should get updated if someone prepends something.

[ssid, 2016/06/20 19:12:06]

Done.

   LAST = DETAILED
 };