Fix edit text rendered with wrong offsets after deletion.

Bug: chromium:835924
Change-Id: I3d6a3d1947cb1b8cda246cc74ef0b79a482b7997
Reviewed-on: https://pdfium-review.googlesource.com/31276
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
diff --git a/xfa/fde/cfde_texteditengine.cpp b/xfa/fde/cfde_texteditengine.cpp
index 1b6e29d..989843c 100644
--- a/xfa/fde/cfde_texteditengine.cpp
+++ b/xfa/fde/cfde_texteditengine.cpp
@@ -821,6 +821,7 @@
   gap_size_ += length;
 
   text_length_ -= length;
+  is_dirty_ = true;
   ClearSelection();
 
   if (delegate_)