win: add a child ProcessReader test

test/win/win_multiprocess.cc

 // Copyright 2015 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 150 matching lines @@
     char c;
     CheckedReadFile(pipe_c2p_read_.get(), &c, sizeof(c));
     ASSERT_EQ(' ', c);
 
     // These have been passed to the child, close our side.
     pipe_c2p_write_.reset();
     pipe_p2c_read_.reset();
 
     WinMultiprocessParent();
 
+    // Close our side of the handles now that we're done. The child can
+    // use this to know when it's safe to complete.
+    pipe_p2c_write_.reset();
+    pipe_c2p_read_.reset();
+
     // Wait for the child to complete.
     ASSERT_EQ(WAIT_OBJECT_0,
               WaitForSingleObject(child_handle_.get(), INFINITE));
 
     DWORD exit_code;
     ASSERT_TRUE(GetExitCodeProcess(child_handle_.get(), &exit_code));
     ASSERT_EQ(exit_code_, exit_code);
   }
 }
 
@@ skipping 32 matching lines @@
     pipe_c2p_write_.reset();
 }
 
 HANDLE WinMultiprocess::ChildProcess() const {
   EXPECT_NE(nullptr, child_handle_.get());
   return child_handle_.get();
 }
 
 }  // namespace test
 }  // namespace crashpad