Add a dummy v8 target to BUILD.gn so we can depend on this from chrome

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/300883003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21566 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

BUILD.gn

@@ -935,3 +935,6 @@ executable("mksnapshot") {
 }
 
 }
+
+group("v8") {
+}