Fix after:
https://codereview.chromium.org/2042253002/
https://codereview.chromium.org/2049743002/

Using the data file is the default now and it shouldn't
be overridden in v8's build_overrides. Also the isolate
logic now needs to include the gni file specifying the
flag.

BUG=chromium:616033
TBR=jochen@chromium.org, vogelheim@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2062363002
Cr-Commit-Position: refs/heads/master@{#36998}