Make it possible to configure synthetic delays through Devtools

tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
index dd52125cbb01b50a0319436ffb173df15bb13523..215731c969994dbddafa5c5bfdb196f59521ae86 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
@@ -216,6 +216,26 @@ class TracingBackend(object):
     self._tracing_data = []
     return result
 
+  def ConfigureSyntheticDelays(self, delays, timeout=10):
+    """Configure synthetic delays for all browser processes.
+
+    Args:
+      delays: A dictionary of delays to configure. Each delay can have the
+              following properties:
+              target_duration: How many seconds the delay point should execute.
+              mode: Delay mode; one of 'static', 'oneshot' or 'alternating'.
+      timeout: Time to wait for operation to complete.
+    """
+    assert not self._IsTracing()
+    req = {
+      'method': 'Tracing.configureSyntheticDelays',
+      'params': {'delays': delays}
+    }
+    self._conn.SendRequest(req, timeout)
+
+    # Wait for the delay configuration acknowledgement synchronously.
+    self._TracingReader()
+
   def _TracingReader(self):
     while self._conn.socket:
       try:
@@ -234,6 +254,8 @@ class TracingBackend(object):
             logging.warning('Unexpected type in tracing data')
         elif 'Tracing.tracingComplete' == res.get('method'):
           break
+        elif 'Tracing.configureSyntheticDelaysComplete' == res.get('method'):
+          break
       except websocket.WebSocketTimeoutException as e:
         logging.warning('Unusual timeout waiting for tracing response (%s).',
                         e)