Fix preparing log file name.

Fix preparing log file name.

Problem:
Excuting with flags as "--prof --logfile-per-isolate --logfile=/path/to/filename"
expected file name: /path/to/isolate-<isolate id>-filename
current result: isolate-<isolate id>-/path/to/filename

This patch makes the file name we expected.

Review URL: https://codereview.chromium.org/960813004

Cr-Commit-Position: refs/heads/master@{#26955}