Make (and verify) public/ files compile under C.

fpdfsdk/src/fpdfview_c_api_test.h

Index: fpdfsdk/src/fpdfview_c_api_test.h
diff --git a/fpdfsdk/src/fpdfview_c_api_test.h b/fpdfsdk/src/fpdfview_c_api_test.h
new file mode 100644
index 0000000000000000000000000000000000000000..755d8215c9e553128c2902aaaf27d32eac699d97
--- /dev/null
+++ b/fpdfsdk/src/fpdfview_c_api_test.h
@@ -0,0 +1,20 @@
+// Copyright 2015 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FPDFSDK_SRC_FPDFVIEW_C_API_TEST_H_
+#define FPDFSDK_SRC_FPDFVIEW_C_API_TEST_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Function to call from gtest harness to ensure linker resolution. Returns
+// 1 on success or 0 on error.
+int CheckPDFiumCApi();
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // FPDFSDK_SRC_FPDFVIEW_C_API_TEST_H_