Add v8_platform to the JS dependencies in the GN build.

This is required for the call to v8::platform::CreateDefaultPlatform.

R=jam@chromium.org

Review URL: https://codereview.chromium.org/540133002

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 9ecd708..f811cf0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -808,6 +808,7 @@
   ]
   deps = [
     "//v8",
+    "//v8:v8_libplatform",
   ]
   forward_dependent_configs_from = [ "//v8" ]
 }