[tracing] Add method to create "weak" global dumps

base/trace_event/process_memory_dump.h

Index: base/trace_event/process_memory_dump.h
diff --git a/base/trace_event/process_memory_dump.h b/base/trace_event/process_memory_dump.h
index 5a6640288170783cd0c77c3580bf5c427dbe3312..606619e63f8ed0110e135b19471165de3774c191 100644
--- a/base/trace_event/process_memory_dump.h
+++ b/base/trace_event/process_memory_dump.h
@@ -98,6 +98,14 @@ class BASE_EXPORT ProcessMemoryDump {
   MemoryAllocatorDump* CreateSharedGlobalAllocatorDump(
       const MemoryAllocatorDumpGuid& guid);
 
+  // Creates a shared MemoryAllocatorDump as CreateSharedGlobalAllocatorDump,
+  // but with a WEAK flag. A weak dump will be removed unless a non-weak dump

[Primiano Tucci (use gerrit), 2016/01/19 17:07:11]

s/removed/discarded/ also below

[ssid, 2016/01/20 18:58:34]

Done.

+  // is created using CreateSharedGlobalAllocatorDump by at least one process
+  // (order of creation does not matter). All owners and children of the removed
+  // dump will also be removed transitively.
+  MemoryAllocatorDump* CreateWeakSharedGlobalAllocatorDump(

[Primiano Tucci (use gerrit), 2016/01/19 17:07:11]

You want to clarify that:
The Weak flag applies only if:
 - No other non-weak dump, with the same GUID, exists already. Otherwise the
dump stays non-weak.
 - No further non-weak dump with the same GUID is created. Otherwise the dump
will become non-weak.

[ssid, 2016/01/20 18:58:34]

Done.

+      const MemoryAllocatorDumpGuid& guid);
+
   // Looks up a shared MemoryAllocatorDump given its guid.
   MemoryAllocatorDump* GetSharedGlobalAllocatorDump(
       const MemoryAllocatorDumpGuid& guid) const;