Only create widgets if really needed.

Currently the GetWidget call will create a backing widget for a control behind
the scenes if it doesn't exist. This can cause more work to be done then needed
as we may need to create the page and any supporting structures. This created
structure will then be torn down as we don't need it anymore once we're done
with the widget.

For the instances where setting the value on the widget will have no effect (as
we'll destroy it anyway) we can tell GetWidget() to not create the widget and
return without doing any work.

BUG=pdfium:632709

Review-Url: https://codereview.chromium.org/2251453002

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