Revert of Revert of Revert of Add d8 target to the GN build.

Revert of Revert of Revert of Add d8 target to the GN build. (patchset #1 id:1 of https://codereview.chromium.org/833563004/)

Reason for revert:
This is now really blocking the roll.  I believe it's just missing

    configs -= [ "//build/config/compiler:chromium_code" ]
    configs += [ "//build/config/compiler:no_chromium_code" ]

in the d8 config but I have no way to check.

Original issue's description:
> 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
>
> Committed: https://crrev.com/ebf3c34fb91b4b61f5c32383a115214c516ab5b0
> Cr-Commit-Position: refs/heads/master@{#25996}

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

Committed: https://crrev.com/316f860a6bd5fcb55ce0cd4a950e829d2b16acea
Cr-Commit-Position: refs/heads/master@{#26050}

[dcarney, 2015-01-14 11:50:49]

Created Revert of Revert of Revert of Add d8 target to the GN build.

[dcarney, 2015-01-14 11:51:53]

The CQ bit was checked by dcarney@chromium.org

[commit-bot: I haz the power, 2015-01-14 11:52:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/810493004/40001

[commit-bot: I haz the power, 2015-01-14 11:52:29]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2015-01-14 11:52:45]

Patchset 2 (id:??) landed as
https://crrev.com/316f860a6bd5fcb55ce0cd4a950e829d2b16acea
Cr-Commit-Position: refs/heads/master@{#26050}