[Telemetry] Move supports_tab_control to PossibleBrowser.

tools/telemetry/telemetry/core/backends/browser_backend.py

Index: tools/telemetry/telemetry/core/backends/browser_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/browser_backend.py b/tools/telemetry/telemetry/core/backends/browser_backend.py
index 2cf61b2c2d26b635f6e04f6121e820c4f4b14e15..ec9786746704ed4b9c1d959c1bcd9a767add111e 100644
--- a/tools/telemetry/telemetry/core/backends/browser_backend.py
+++ b/tools/telemetry/telemetry/core/backends/browser_backend.py
@@ -15,11 +15,9 @@ class ExtensionsNotSupportedException(Exception):
 class BrowserBackend(object):
   """A base class for browser backends."""
 
-  def __init__(self, is_content_shell, supports_extensions, browser_options,
-               tab_list_backend):
+  def __init__(self, supports_extensions, browser_options, tab_list_backend):
     assert browser_options.browser_type
     self.browser_type = browser_options.browser_type
-    self.is_content_shell = is_content_shell
     self._supports_extensions = supports_extensions
     self.browser_options = browser_options
     self._browser = None