[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..ad07e5d5ed2fb7ad3e1b5996ada7b348971912e9 100644
--- a/base/trace_event/memory_dump_manager.cc
+++ b/base/trace_event/memory_dump_manager.cc
@@ -8,6 +8,7 @@
 
 #include "base/atomic_sequence_num.h"
 #include "base/compiler_specific.h"
+#include "base/hash.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 +158,7 @@ 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 +434,12 @@ void MemoryDumpManager::OnTraceLogDisabled() {
   session_state_ = nullptr;
 }
 
+// static
+int MemoryDumpManager::ChildProcessIdToTracingProcessId(int child_process_id) {
+  return Hash(reinterpret_cast<const char*>(&child_process_id),

[dcheng, 2015/06/24 20:34:06]

What happens if this just happens to be -1?

[ssid, 2015/06/25 02:35:49]

Please see ChildProcessHost::kInvalidUniqueID, and the comment on
GenerateChildProcessUniqueId(). It only generates id starting from 1 and
increments, only positive values. So, id generated for a child process can't be
-1, even in single-process mode.
But this relies on the fact that kInvalidUniqueID is defined as -1!

[dcheng, 2015/06/25 02:45:39]

I don't understand: how are you guaranteeing that no integer value of
child_process_id hashes to -1?

[ssid, 2015/06/25 04:04:09]

I see what you mean now. Thanks I just realized I forgot to change the constant
corresponding to the hash value. Fixed it now.

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