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

third_party/protoc.gypi

@@ -17,9 +17,11 @@
       # protobuf_lib is a gyp target.
       'dependencies': [
         '<(protobuf_lib)',
+        '<(protoc_host_target)',
       ],
       'export_dependent_settings': [
         '<(protobuf_lib)',
+        '<(protoc_host_target)',
       ],
     }],
     ['protobuf_lib_type=="source"', {