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/cpdfsdk_pageview.h b/fpdfsdk/cpdfsdk_pageview.h
index 8bede08..9eebb6e 100644
--- a/fpdfsdk/cpdfsdk_pageview.h
+++ b/fpdfsdk/cpdfsdk_pageview.h
@@ -60,6 +60,8 @@
     return m_pFormFillEnv.Get();
   }
 
+  CFX_WideString GetSelectedText();
+
   bool OnLButtonDown(const CFX_PointF& point, uint32_t nFlag);
   bool OnLButtonUp(const CFX_PointF& point, uint32_t nFlag);
 #ifdef PDF_ENABLE_XFA