Adding instrumentation to locate the source of jankiness.

storage/browser/blob/local_file_stream_reader.cc

Index: storage/browser/blob/local_file_stream_reader.cc
diff --git a/storage/browser/blob/local_file_stream_reader.cc b/storage/browser/blob/local_file_stream_reader.cc
index e6403b7b2275219cb48a07898db4977d2dd1b2e7..2ddff12708310e88f06cb8084042e0bc66f3742e 100644
--- a/storage/browser/blob/local_file_stream_reader.cc
+++ b/storage/browser/blob/local_file_stream_reader.cc
@@ -8,6 +8,7 @@
 #include "base/files/file_util_proxy.h"
 #include "base/location.h"
 #include "base/logging.h"
+#include "base/profiler/scoped_profile.h"
 #include "base/task_runner.h"
 #include "net/base/file_stream.h"
 #include "net/base/io_buffer.h"
@@ -101,6 +102,11 @@ void LocalFileStreamReader::DidVerifyForOpen(
 void LocalFileStreamReader::DidOpenFileStream(
     const net::CompletionCallback& callback,
     int result) {
+  // TODO(vadimt): Remove ScopedProfile below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedProfile tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 LocalFileStreamReader::DidOpenFileStream"));
+
   if (result != net::OK) {
     callback.Run(result);
     return;