Add GN version of blink_unittests target.
Add GN version of blink_unittests target.

This is needed to avoid colision (specially in GN) with Chromium's
blink_tests targets.

See https://codereview.chromium.org/1118863002/ for how this came along.

Tested on Linux with the following command lines:

$ gn gen out-gn/Debug
$ ninja -C out-gn/Debug both_gn_and_gyp

BUG=483171
TEST=see above
TBR=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1122693002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194849 bbb929c8-8fbe-4397-9dbb-9b2b20218538

XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk