Add FORM_DeleteSelectedText() and embedder tests.

This method deletes the current text selection in a form text field or
user-editable form combobox text field. If there is no selection, this
method does nothing.

BUG=chromium:59266

Change-Id: I3229ffad990c62beac1cf769cd366458b9ee5daa
Reviewed-on: https://pdfium-review.googlesource.com/8370
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Diana Gage <drgage@google.com>
diff --git a/fpdfsdk/cpdfsdk_annothandlermgr.cpp b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
index 9e649f6..fd77a73 100644
--- a/fpdfsdk/cpdfsdk_annothandlermgr.cpp
+++ b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
@@ -81,6 +81,10 @@
   return GetAnnotHandler(pAnnot)->GetSelectedText(pAnnot);
 }
 
+void CPDFSDK_AnnotHandlerMgr::Annot_DeleteSelectedText(CPDFSDK_Annot* pAnnot) {
+  GetAnnotHandler(pAnnot)->DeleteSelectedText(pAnnot);
+}
+
 IPDFSDK_AnnotHandler* CPDFSDK_AnnotHandlerMgr::GetAnnotHandler(
     CPDFSDK_Annot* pAnnot) const {
   return GetAnnotHandler(pAnnot->GetAnnotSubtype());