telemetry: Add a method to access tabs by key in TabList.

tools/telemetry/telemetry/core/tab_list.py

Index: tools/telemetry/telemetry/core/tab_list.py
diff --git a/tools/telemetry/telemetry/core/tab_list.py b/tools/telemetry/telemetry/core/tab_list.py
index 06cb667e1f6a3aa2dbacc51106f198afa92fce2f..da83d699d3968b12a22095eb4b8d2710a3d0c3e6 100644
--- a/tools/telemetry/telemetry/core/tab_list.py
+++ b/tools/telemetry/telemetry/core/tab_list.py
@@ -16,3 +16,8 @@ class TabList(object):
 
   def __getitem__(self, index):
     return self._tab_list_backend.__getitem__(index)
+
+  def GetTabById(self, identifier):
+    """The identifier of a tab can be accessed with tab.id."""
+    return self._tab_list_backend.GetTabById(identifier)
+