Stop adding linux_site_isolation to some CQ jobs.

Persistent failures on linux_site_isolation are not visible to Chromium
Sheriffs, because this bot is not part of the main waterfall.  Until
this happens, this bot should not be automatically added to CQ jobs.

Bug: 786555
Change-Id: Ie6455ae5996413211ff626c8db4fa5692411cf0b
Reviewed-on: https://chromium-review.googlesource.com/777736Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517627}

content/browser/frame_host/PRESUBMIT.py

-# Copyright 2016 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.
-"""Presubmit script for //content/browser/frame_host.
-
-See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
-for more details about the presubmit API built into depot_tools.
-"""
-
-def PostUploadHook(cl, change, output_api):
-  """git cl upload will call this hook after the issue is created/modified.
-
-  This hook adds extra try bots to the CL description in order to run site
-  isolation tests in addition to CQ try bots.
-  """
-  return output_api.EnsureCQIncludeTrybotsAreAdded(
-    cl,
-    [
-      'master.tryserver.chromium.linux:linux_site_isolation'
-    ],
-    'Automatically added site isolation trybots to run tests on CQ.')
-