[Swarming] Isolate sancov_merger.

tools/testrunner/testrunner.isolate

 # Copyright 2015 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'variables': {
     'command': [
       '../run-tests.py',
     ],
     'files': [
       '../run-tests.py',
       './'
     ],
   },
   'conditions': [
     ['coverage==1 and sanitizer_coverage=="bb"', {
       'variables': {
         'files': [
+          '../sanitizers/sancov_merger.py',
           '../../third_party/llvm/projects/compiler-rt/lib/sanitizer_common/scripts/sancov.py',
         ],
       },
     }],
   ],
 }