[tracing] Send unique tracing process id for cross-process memory dumps identification.

base/trace_event/memory_dump_manager.cc

Index: base/trace_event/memory_dump_manager.cc
diff --git a/base/trace_event/memory_dump_manager.cc b/base/trace_event/memory_dump_manager.cc
index 3814c21fd28698babd28008c41ca85283b3e817f..414c458af195775ddadcabc145fba0ecc6e17b1f 100644
--- a/base/trace_event/memory_dump_manager.cc
+++ b/base/trace_event/memory_dump_manager.cc
@@ -8,6 +8,8 @@
 
 #include "base/atomic_sequence_num.h"
 #include "base/compiler_specific.h"
+#include "base/hash.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/thread_task_runner_handle.h"
 #include "base/trace_event/memory_dump_provider.h"
 #include "base/trace_event/memory_dump_session_state.h"
@@ -157,6 +159,7 @@ void MemoryDumpManager::SetInstanceForTesting(MemoryDumpManager* instance) {
 MemoryDumpManager::MemoryDumpManager()
     : delegate_(nullptr),
       memory_tracing_enabled_(0),
+      tracing_process_id_(kInvalidTracingProcessId),
       skip_core_dumpers_auto_registration_for_testing_(false) {
   g_next_guid.GetNext();  // Make sure that first guid is not zero.
 }
@@ -432,6 +435,11 @@ void MemoryDumpManager::OnTraceLogDisabled() {
   session_state_ = nullptr;
 }
 
+// static
+int MemoryDumpManager::ChildProcessIdToTracingProcessId(int child_process_id) {
+  return Hash(IntToString(child_process_id));

[Primiano Tucci (use gerrit), 2015/06/24 17:47:35]

Ok for the Hash, but do:
Hash(reinterpret_cast<const char*>(&child_process_id),
sizeof(child_process_id));
as that will spread out the state space more.
I tested it and got the first collision after 260k elements, which should be
good enough.
If you convert to ascii, the state space is too small and collides more.

+}
+
 MemoryDumpManager::MemoryDumpProviderInfo::MemoryDumpProviderInfo(
     const scoped_refptr<SingleThreadTaskRunner>& task_runner)
     : task_runner(task_runner), consecutive_failures(0), disabled(false) {