Explicit dependency on executables in proto_library.gni.

GN uses wrapper script to invoke protobuf compiler and
script depends on it using deps variable in GN action.
This change makes this script also dependent on protoc
(or plugin) executable explicitly.

BUG=644525

Review-Url: https://chromiumcodereview.appspot.com/2427943002
Cr-Commit-Position: refs/heads/master@{#426374}
1 file changed