Add d8 target to BUILD.gn
Add d8 target to BUILD.gn

R=machenbach@chromium.org,brettw@chromium.org
LOG=n
BUG=none

Review URL: https://codereview.chromium.org/846743004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26151}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh