Add FORM_GetSelectedText() function.

This function copies the selected text from a form text field or
form combobox text field into the buffer parameter and returns the
length of the selected text string. When buffer is a nullptr or
buflen is less than the length of the selected text, this function
does not modify the buffer and only returns the selected text length.

BUG=chromium:59266

Change-Id: Ie77de38e45bbe6f9ea033826c961435304eedfc7
Reviewed-on: https://pdfium-review.googlesource.com/6413
Reviewed-by: dsinclair <dsinclair@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/fpdfview_c_api_test.c b/fpdfsdk/fpdfview_c_api_test.c
index 0deaf9d..05242c6 100644
--- a/fpdfsdk/fpdfview_c_api_test.c
+++ b/fpdfsdk/fpdfview_c_api_test.c
@@ -156,6 +156,7 @@
     CHK(FORM_OnKeyDown);
     CHK(FORM_OnKeyUp);
     CHK(FORM_OnChar);
+    CHK(FORM_GetSelectedText);
     CHK(FORM_ForceToKillFocus);
     CHK(FPDFPage_HasFormFieldAtPoint);
     CHK(FPDFPage_FormFieldZOrderAtPoint);