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

base/trace_event/memory_allocator_dump_guid.cc

Index: base/trace_event/memory_allocator_dump_guid.cc
diff --git a/base/trace_event/memory_allocator_dump_guid.cc b/base/trace_event/memory_allocator_dump_guid.cc
index a4ea50d416a0d5bd784e7c74501f77ad45d7b49f..2b5c1bba27d07deeb414714e71a997a29689537c 100644
--- a/base/trace_event/memory_allocator_dump_guid.cc
+++ b/base/trace_event/memory_allocator_dump_guid.cc
@@ -6,9 +6,25 @@
 
 #include "base/format_macros.h"
 #include "base/hash.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 
 namespace base {
+namespace {
+int unique_child_process_id = -1;

[Primiano Tucci (use gerrit), 2015/06/19 13:47:49]

nit: g_ prefix
Add also a kInvalidChildProcessId = -1 (See below for the reason)

[ssid, 2015/06/19 14:51:19]

Done.

+
+uint64 InsertChildIdAndHash(std::string guid_str) {

[Primiano Tucci (use gerrit), 2015/06/19 13:47:49]

nit: s/InsertChildIdAndHash/ExpandChildProcessIdAndHash/

[ssid, 2015/06/19 14:51:19]

Done.

+  const char kReplaceChildIdStr[] = "$$";
+  size_t pos = guid_str.find(kReplaceChildIdStr);
+  if (pos != std::string::npos) {
+    DCHECK_NE(unique_child_process_id, -1);

[Primiano Tucci (use gerrit), 2015/06/19 13:47:49]

use kInvalid... instead of -1

[ssid, 2015/06/19 14:51:19]

Done.

+    guid_str.replace(pos, sizeof(kReplaceChildIdStr) - 1,
+                     IntToString(unique_child_process_id));
+  }
+  return Hash(guid_str);
+}
+}  // namespace
+
 namespace trace_event {
 
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(uint64 guid) : guid_(guid) {
@@ -19,7 +35,12 @@ MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid()
 }
 
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(const std::string& guid_str)
-    : MemoryAllocatorDumpGuid(Hash(guid_str)) {
+    : MemoryAllocatorDumpGuid(InsertChildIdAndHash(guid_str)) {
+}
+
+// static
+void MemoryAllocatorDumpGuid::SetUniqueChildProcessId(int child_process_id) {
+  unique_child_process_id = child_process_id;

[Primiano Tucci (use gerrit), 2015/06/19 13:47:49]

can you add a 

DCHECK(child_process_id_ == kInvalidChildProcessId || child_process_id ==
kInvalidChildProcessId)
essentially this checks that the CPID can be either set or reset to invalid, but
not changed.

[ssid, 2015/06/19 14:51:19]

Done.

 }
 
 std::string MemoryAllocatorDumpGuid::ToString() const {