This prevents the flag from being set from e.g. Chromium. Instead, just use
relative paths like everything else in the build system.

Bug: chromium:825347, v8:7601

Change-Id: I080d9999b0b63bafc2c1978f70322eb48814a3b8
Reviewed-on: https://chromium-review.googlesource.com/980557
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52263}