This makes the symbolizer path relative, as the absolute paths contain
a drive letter + colon on windows. The colon is confused by the
sanitizer as an option separator.

The test driver changes the cwd to the V8 root dir in each
invocation.

Bug: chromium:726584
Change-Id: Icf4e5a55bba5dec8e59a3dfe3eccdf7224e65c33
Reviewed-on: https://chromium-review.googlesource.com/721124Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48652}