[test] Fully fix timeout-scalefactor for numfuzzer

TBR=majeski@google.com
NOTRY=true

Bug: v8:6917
Change-Id: I8ca2297ab9641b81a91ee7f8ff594bbe4b9e369b
Reviewed-on: https://chromium-review.googlesource.com/895366Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51005}

tools/testrunner/num_fuzzer.py

@@ -191,7 +191,7 @@ class NumFuzzer(base_runner.BaseTestRunner):
                           self.mode_options.execution_mode,
                           self.outdir,
                           self.mode_options.flags, options.verbose,
-                          options.timeout * self.timeout_scalefactor(options),
+                          options.timeout * self._timeout_scalefactor(options),
                           options.isolates,
                           options.command_prefix,
                           options.extra_flags,