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

The memory shared between browser and renderer must be identified with
the same unique id for memory dumps in tracing. Currently the child
process is not aware of its unique child process id. But, the child
process id cannot be exposed in the child process (see discussion in
crrev.com/1155683009).

So, a hash of the child process id allocated by the browser process is
sent to child process along with begin tracing message and will be
exposed in the child process MemoryDumpManager. The allocator that
needs to create a global guid will use the id to identify memory dumps.

See the design doc https://goo.gl/ncMfUV for more details.

BUG=497726

Review URL: https://codereview.chromium.org/1173263004

Cr-Commit-Position: refs/heads/master@{#336210}

1 files changed, 3 insertions, 2 deletions

diff --git a/components/tracing/tracing_messages.h b/components/tracing/tracing_messages.h
index ebdefc1..dd14b90 100644
--- a/components/tracing/tracing_messages.h
+++ b/components/tracing/tracing_messages.h
@@ -32,9 +32,10 @@ IPC_ENUM_TRAITS_MAX_VALUE(
     static_cast<int>(base::trace_event::MemoryDumpType::LAST))
 
 // Sent to all child processes to enable trace event recording.
-IPC_MESSAGE_CONTROL2(TracingMsg_BeginTracing,
+IPC_MESSAGE_CONTROL3(TracingMsg_BeginTracing,
                      std::string /*  trace_config_str */,
-                     base::TraceTicks /* browser_time */)
+                     base::TraceTicks /* browser_time */,
+                     int /* Tracing process id (hash of child id) */)
 
 // Sent to all child processes to disable trace event recording.
 IPC_MESSAGE_CONTROL0(TracingMsg_EndTracing)