Update a comment in proto_library.gni
Update a comment in proto_library.gni

The code for perfetto_proto_library template has "cpp" as one of the
elements of default value. This CL fixes a comment to reflect that.

Change-Id: I834720507a82b10e5508fea85ebe0328031c7ace
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto