Stub out Document::syncAnnotScan method.chromium/2834

The PDF specification [1] says:

"
syncAnnotScan guarantees that all annotations will be scanned
by the time this method returns.
(..)
Normally a background task runs that examine every page and
looks for annotations during idle times.
"

The statement details specifically how Acrobat implements
this method.
Although, neither the method itself nor the background scanner
task are implemented in PDFium (as of today, Ago/2016),
not having ::syncAnnotScan at least stubbed out can be considered
harmfull since its absence makes JS acrobat scripts silently
fail when it has a call to it.

Given that, and following a stub-out pattern present in other
methods including ::addAnnot and ::addField, CL provides
a stubbed out implementation of Document::syncAnnotScan.

[1] http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/js_api_reference.pdf

BUG=pdfium:492

Review-Url: https://codereview.chromium.org/2265553002

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