Add FORM_ReplaceSelection() and embedder tests.

This method replaces the selected text in a user-editable form text
area with another text string (which can be empty or non-empty). If
there is no selected text, FORM_ReplaceSelection() will append the
replacement text after the current caret position.

BUG=chromium:59266

Change-Id: I76448ef757d107888c33ebd5656457ebac93b952
Reviewed-on: https://pdfium-review.googlesource.com/8812
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Diana Gage <drgage@google.com>
diff --git a/fpdfsdk/fpdfview_c_api_test.c b/fpdfsdk/fpdfview_c_api_test.c
index 95a0175..7d42be8 100644
--- a/fpdfsdk/fpdfview_c_api_test.c
+++ b/fpdfsdk/fpdfview_c_api_test.c
@@ -182,7 +182,7 @@
     CHK(FORM_OnKeyUp);
     CHK(FORM_OnChar);
     CHK(FORM_GetSelectedText);
-    CHK(FORM_DeleteSelectedText);
+    CHK(FORM_ReplaceSelection);
     CHK(FORM_ForceToKillFocus);
     CHK(FPDFPage_HasFormFieldAtPoint);
     CHK(FPDFPage_FormFieldZOrderAtPoint);