Fix flaky TabListBackendTest

The tests should only run when tab control is supported.

See log on FYI bot Android Tests (K N5, user build)(dbg):
http://build.chromium.org/p/chromium.fyi/builders/Android%20Tests%20%28K%20N5%2C%20user%20build%29%28dbg%29/builds/3050/steps/telemetry_unittests/logs/stdio

BUG=437881

Review URL: https://codereview.chromium.org/979833003

Cr-Commit-Position: refs/heads/master@{#319103}

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

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
+
+from telemetry import decorators
 from telemetry.core import exceptions
 from telemetry.unittest_util import tab_test_case
 
 
 class TabListBackendTest(tab_test_case.TabTestCase):
+  @decorators.Enabled('has tabs')
   def testTabIdMatchesContextId(self):
     # Ensure that there are two tabs.
     while len(self.tabs) < 2:
@@ -18,6 +21,7 @@ class TabListBackendTest(tab_test_case.TabTestCase):
       self.assertEquals(tab.id, context_id)
       tabs.append(self.tabs.GetTabById(context_id))
 
+  @decorators.Enabled('has tabs')
   def testTabIdStableAfterTabCrash(self):
     # Ensure that there are two tabs.
     while len(self.tabs) < 2: