Fix bug where Chromium crashes in PDFiumEngine::OnMouseUp().

pdf/pdfium/pdfium_engine.cc

Index: pdf/pdfium/pdfium_engine.cc
diff --git a/pdf/pdfium/pdfium_engine.cc b/pdf/pdfium/pdfium_engine.cc
index eb7f4cb08fb315aac8cb6540a857b39c3212fd21..ab53670eae00e6efabf1e77e9b88beaf50c5ce7c 100644
--- a/pdf/pdfium/pdfium_engine.cc
+++ b/pdf/pdfium/pdfium_engine.cc
@@ -1857,7 +1857,7 @@ bool PDFiumEngine::OnMouseUp(const pp::MouseInputEvent& event) {
     FORM_OnLButtonUp(form_, pages_[page_index]->GetPage(), 0, page_x, page_y);
   }
 
-  if (area == PDFiumPage::FORM_TEXT_AREA)
+  if (area == PDFiumPage::FORM_TEXT_AREA && last_page_mouse_down_ != -1)

[Lei Zhang, 2017/06/27 01:26:51]

This will definitely fix the crash, but another question that arises is whether
we should be using |last_page_mouse_down_|, or |page_index|.

From looking at the code:
- |last_page_mouse_down_| is really "last_page_left_mouse_button_down_".
- On a multi-page PDF, if the left mouse down occurs on page 2, the mouse button
is held and dragged to page 1, and then released on page 1, then
|last_page_mouse_down_| is 1 and |page_index| is 0. (Both variables are 0-based)

[dsinclair, 2017/06/27 15:24:21]

FORM_GetSelectedText returns the text from a text field or combo box which can't
go over page boundaries. I'd hope that would mean last_page_mouse_down_ and
page_index_ should match, otherwise, I'm not sure what that would mean?

     SetFormSelectedText(form_, pages_[last_page_mouse_down_]->GetPage());
 
   if (!selecting_)