Revert of Removing browser.Start() and instead starts the browser at the point of creation.

tools/telemetry/telemetry/core/extension_unittest.py

Index: tools/telemetry/telemetry/core/extension_unittest.py
diff --git a/tools/telemetry/telemetry/core/extension_unittest.py b/tools/telemetry/telemetry/core/extension_unittest.py
index 77e44e382af9b2ddf184557f08dac8963d1b94cf..23cac65334bda36c1cdfca0b12e4f82e6e3830bd 100644
--- a/tools/telemetry/telemetry/core/extension_unittest.py
+++ b/tools/telemetry/telemetry/core/extension_unittest.py
@@ -32,6 +32,7 @@
       # May not find a browser that supports extensions.
       return False
     self._browser = browser_to_create.Create()
+    self._browser.Start()
     self._extension = self._browser.extensions[load_extension]
     self._extension_id = load_extension.extension_id
     self.assertTrue(self._extension)
@@ -123,6 +124,7 @@
     # May not find a browser that supports extensions.
     if browser_to_create:
       self._browser = browser_to_create.Create()
+      self._browser.Start()
 
   def tearDown(self):
     if self._browser: