[Swarming] Isolate sancov_merger.

BUG=chromium:535160, chromium:568949
LOG=n
TBR=tandrii@chromium.org, kjellander@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1777263002

Cr-Commit-Position: refs/heads/master@{#34671}

tools/testrunner/testrunner.isolate

@@ -15,6 +15,7 @@
     ['coverage==1 and sanitizer_coverage=="bb"', {
       'variables': {
         'files': [
+          '../sanitizers/sancov_merger.py',
           '../../third_party/llvm/projects/compiler-rt/lib/sanitizer_common/scripts/sancov.py',
         ],
       },