[CT] Use zips of SKPs to speed up "isolate tests"

chrome/ct_skps.isolate

 # Copyright (c) 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.
 
 {
   'conditions': [
     ['OS=="linux"', {
       'variables': {
         'files': [
           '../../skia/resources/',
           '../../skia/out/<(CONFIGURATION)/<(TOOL_NAME)',
           '../content/test/ct/run_ct_skps.py',
-          '../../skps/slave<(SLAVE_NUM)/',
+          '../../skps/slave<(SLAVE_NUM).zip',
         ],
         'command': [
           'python',
           '../content/test/ct/run_ct_skps.py',
           '--slave_num', '<(SLAVE_NUM)',
           '--tool', '<(TOOL_NAME)',
           '--git_hash', '<(GIT_HASH)',
           '--isolated_outdir', '${ISOLATED_OUTDIR}',
           '--configuration', '<(CONFIGURATION)',
         ],
       },
     }],
   ]
 }