Remove isolate_file variable from breakpad unittest gyp.
Remove isolate_file variable from breakpad unittest gyp.

This isolate file doesn't appear to exist anyone. Breakpad
unittests are failing due to not being able to find this file.

TBR=thestig@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1898193006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388330}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk