enable -Wunused-private-field for linux and chromeos.

chrome/browser/chromeos/process_proxy/process_proxy_browsertest.cc

Index: chrome/browser/chromeos/process_proxy/process_proxy_browsertest.cc
diff --git a/chrome/browser/chromeos/process_proxy/process_proxy_browsertest.cc b/chrome/browser/chromeos/process_proxy/process_proxy_browsertest.cc
index ca2bd368c96556b5e38dcf69af8be306a283b034..131f9de4420c0a8ff968ec0042702763a4a42820 100644
--- a/chrome/browser/chromeos/process_proxy/process_proxy_browsertest.cc
+++ b/chrome/browser/chromeos/process_proxy/process_proxy_browsertest.cc
@@ -175,9 +175,6 @@ class SigIntTestRunner : public TestRunner {
     // Send SingInt and verify the process exited.
     EXPECT_TRUE(registry->SendInput(pid_, "\003"));
   }
-
- private:
-  bool output_received_;
 };
 
 }  // namespace