Add py_trace_event to DEPS.

py_trace_event lets you instrument Python code to get Chrome traces.
In third_party without a README.chromium, because
it's by Chromium authors and Chromium-licensed.

BUG=None.
TEST=gclient sync

Review URL: https://codereview.chromium.org/615363002

Cr-Commit-Position: refs/heads/master@{#297736}
diff --git a/DEPS b/DEPS
index 3eb25c9..72f971ed 100644
--- a/DEPS
+++ b/DEPS
@@ -300,6 +300,9 @@
 
   'src/third_party/boringssl/src':
    'https://boringssl.googlesource.com/boringssl.git' + '@' +  Var('boringssl_revision'),
+
+  'src/third_party/py_trace_event/src':
+    Var('chromium_git') + '/external/py_trace_event.git' + '@' + 'dd463ea9e2c430de2b9e53dea57a77b4c3ac9b30',
 }