Fix protoc dependency race.

According to Eureka team, using protobuf_lib_type=="target" without a
direct dependency on a protoc target causes flakiness in the build.
This introduces a new variable to represent the protoc host target,
with a default value for platforms like iOS where a prebuilt copy of
protoc is used.

Change-Id: I39c08bd76c6bc4a5291b0e19b2d22a9f3beb04d0

cdm/cdm.gyp

@@ -104,5 +104,9 @@
         'wvcdm_static',
       ],
     },
+    {
+      'target_name': 'dummy',
+      'type': 'none',
+    }
   ],
 }