[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..9a7362e9174304697c9e1adaad96c00fa25af302 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 +434,11 @@ 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 19:05:36]

Don't you need to pass a length here? This is going to call the const
std::string& version since std::string is implicitly convertible from char*.
std::string's implicit conversion is going to read through bytes until it finds
a \0, which is not necessarily in [&child_process_id, &child_process_id +
sizeof(child_process_id))

[ssid, 2015/06/24 19:11:07]

Done.

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