API to provide all named destinations and associated page numbers

[GoogleCodeExporter]

What steps will reproduce the problem?
Currently, there is no API in pdfium that provides all the named destinations 
and the associated page numbers. 

There is an existing API - PDFiumEngine::GetNamedDestinationPage() - that 
returns page number for a particular destination. But to support nameddest open 
pdf parameter in OOP PDF, using this API can cause race condition while setting 
up the viewport because of use of asynchronous messages between plugin and page.

Another possible use case (for example) for such an API is when we want to 
extend UI and show list of all named destinations.

For more info, please see :
https://codereview.chromium.org/535823005/

What is the expected output? What do you see instead?
It will be useful to have such an API as there is none as of now.

What version of the product are you using? On what operating system?
Linux/Chrome/39.0.2166.0

Please provide any additional information below.

Original issue reported on code.google.com by n.ban...@samsung.com on 23 Sep 2014 at 2:00

[GoogleCodeExporter]

bo_xu@ - Do you think it's possible to add an API to pdfium that returns all 
named destinations and associated page numbers? Please let me know WDYT :)

Original comment by n.ban...@samsung.com on 7 Oct 2014 at 11:48