Add embeddertest for form text rendering and saving

This CL adds an embeddertest that adds text to a textfield and saves it. It
also adds a new 'charcode' option for .evt files in pdfium_test.

Change-Id: I14fbf50e2b1d5ae0bdc68d1dd25dc4f889c49bfb
Reviewed-on: https://pdfium-review.googlesource.com/4150
Commit-Queue: Nicolás Peña <npm@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/testing/test_support.h b/testing/test_support.h
index b734bc3..f175811 100644
--- a/testing/test_support.h
+++ b/testing/test_support.h
@@ -115,6 +115,12 @@
   void ClearString();
   const std::string& GetString() const { return m_String; }
 
+ protected:
+  static int GetBlockFromString(void* param,
+                                unsigned long pos,
+                                unsigned char* buf,
+                                unsigned long size);
+
  private:
   static int WriteBlockCallback(FPDF_FILEWRITE* pFileWrite,
                                 const void* data,