Lazy generate an "AP" when an Annot's hidden state changes

Now that Document::getAnnot works and annotation instances
can have its properties changed, consider the following
scenario:

- A PDF content has an annotation without AP and
CPVT_GenerateAP is called to generate one.
- However the annotation also has its hidden flag set (/F 2),
and CPVT_GenerateAP bails out earlier, not generating an AP.
- When the PDF's Javascript runs, it acquires an instance of
this annotation object, bounded to JS using Document::getAnnot(),
and set its "hidden" flag to false.
- At this point, the annotation should get drawn, but it does
not because its "AP" was never generated.

CL fixes this scenario by making PDFium able to lazy
generate APs, if needed.

BUG=pdfium:492

Review-Url: https://codereview.chromium.org/2265313002

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