[GN] Output external snapshot blobs in out directory.

The snapshot and natives blob files should be output in the out directory
instead of the gen directory so that they can be picked up by the
executable.

BUG=421063
LOG=N

Review URL: https://codereview.chromium.org/805813004

Cr-Commit-Position: refs/heads/master@{#25827}

BUILD.gn

@@ -280,7 +280,7 @@ if (v8_use_external_startup_data) {
     ]
 
     outputs = [
-      "$root_gen_dir/natives_blob.bin"
+      "$root_out_dir/natives_blob.bin"
     ]
 
     script = "tools/concatenate-files.py"
@@ -333,10 +333,10 @@ action("run_mksnapshot") {
   }
 
   if (v8_use_external_startup_data) {
-    outputs += [ "$root_gen_dir/snapshot_blob.bin" ]
+    outputs += [ "$root_out_dir/snapshot_blob.bin" ]
     args += [
       "--startup_blob",
-      rebase_path("$root_gen_dir/snapshot_blob.bin", root_build_dir)
+      rebase_path("$root_out_dir/snapshot_blob.bin", root_build_dir)
     ]
   }
 }