Fix the confict of declaration and definition between struct and class

This fixes the error when compiling with clang syntax checking
The error message was:
/update_engine/test_utils.h:188:1: error: 'ObjectFeederAction'
defined as a struct template here but previously declared as a
class template [-Werror,-Wmismatched-tags]

BUG=chromium:218781
TEST=USE="chrome_internal" CXXFLAGS="-clang -print-cmdline"
     CFLAGS="-clang -print-cmdline" emerge-x86-alex update_engine

Change-Id: Ieac9981c84c4e5779cbf876650f21a5d1b2acd72
Reviewed-on: https://gerrit.chromium.org/gerrit/47476
Reviewed-by: Jay Srinivasan <jaysri@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
diff --git a/action_pipe_unittest.cc b/action_pipe_unittest.cc
index b0a96b7..0f777fb 100644
--- a/action_pipe_unittest.cc
+++ b/action_pipe_unittest.cc
@@ -23,7 +23,8 @@
 };
 
 // This is a simple Action class for testing.
-struct ActionPipeTestAction : public Action<ActionPipeTestAction> {
+class ActionPipeTestAction : public Action<ActionPipeTestAction> {
+ public:
   typedef string InputObjectType;
   typedef string OutputObjectType;
   ActionPipe<string>* in_pipe() { return in_pipe_.get(); }