Make it possible to configure synthetic delays through Devtools

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

Index: tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py b/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
index e1fdbb589cfc154157565996a0bffb5f96c8f7db..74daa79cd4daf2ba039082bf7988e30131da3921 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
@@ -249,6 +249,10 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
   def supports_tracing(self):
     return self.is_content_shell or self.chrome_branch_number >= 1385
 
+  def _EnsureTracingBackend(self):
+    if self._tracing_backend is None:
+      self._tracing_backend = tracing_backend.TracingBackend(self._port)
+
   def StartTracing(self, custom_categories=None,
                    timeout=web_contents.DEFAULT_WEB_CONTENTS_TIMEOUT):
     """ custom_categories is an optional string containing a list of
@@ -257,8 +261,7 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
     "webkit,cc,disabled-by-default-cc.debug" to trace only those three
     event categories.
     """
-    if self._tracing_backend is None:
-      self._tracing_backend = tracing_backend.TracingBackend(self._port)
+    self._EnsureTracingBackend()
     return self._tracing_backend.StartTracing(custom_categories, timeout)
 
   def StopTracing(self):
@@ -275,6 +278,15 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
         self._tracing_backend.AddTabToMarkerMapping(tab, debugger_url)
     return self._tracing_backend.StopTracing()
 
+  @property
+  def supports_synthetic_delays(self):
+    return self.is_content_shell or self.chrome_branch_number >= 1700
+
+  def ConfigureSyntheticDelays(self, delays,
+      timeout=web_contents.DEFAULT_WEB_CONTENTS_TIMEOUT):
+    self._EnsureTracingBackend()
+    return self._tracing_backend.ConfigureSyntheticDelays(delays, timeout)
+
   def GetProcessName(self, cmd_line):
     """Returns a user-friendly name for the process of the given |cmd_line|."""
     if not cmd_line: