Revert of Revert of Add d8 target to the GN build. (patchset #1 id:1 of https://codereview.chromium.org/838983002/)

Reason for revert:
Relanding because this is not actually blocking the roll.

Original issue's description:
> Revert of Add d8 target to the GN build. (patchset #2 id:40001 of https://codereview.chromium.org/834113005/)
>
> Reason for revert:
> V8 roll fails due to this change. (see https://codereview.chromium.org/843673003/)
>
> Original issue's description:
> > Add d8 target to the GN build.
> >
> > Also formats the rest of the file with "gn format". This accounts for all the changes except for the "d8" target additions.
> >
> > Committed: https://crrev.com/36383f08c1f692ea797d54a27d7c6b3d3d53b440
> > Cr-Commit-Position: refs/heads/master@{#25984}
>
> TBR=jochen@chromium.org,machenbach@chromium.org,brettw@chromium.org
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/ef50fdfdb05e1a3a3822962f5235b804218a5f4e
> Cr-Commit-Position: refs/heads/master@{#25985}

TBR=jochen@chromium.org,machenbach@chromium.org,brettw@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/833563004

Cr-Commit-Position: refs/heads/master@{#25996}