Delete breakpad_unittest.isolate (no longer needed)

As of https://codereview.chromium.org/1854233002/

BUG=none

Review URL: https://codereview.chromium.org/1892443002

Cr-Original-Commit-Position: refs/heads/master@{#387388}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: de9a653bdb0fcf0248f405bd7772a3113d39e542