pdfium: Fix all -Wdelete-non-virtual-dtor violations on Mac.

Calling `delete` on an object of a type that has virtual functions but
not a virtual destructor is questionable: Since the object has virtual functions,
it likely has subclasses, so if it's deleted through the base pointer and the
destructor isn't virtual, the subclass destructor won't be called.

In most cases, the classes getting deleted can just be marked final to tell
the compiler that it can't possibly have subclasses (this also enables the
compiler to generate better code).

Two classes didn't have any sub- or superclasses but virtual functions -
this doesn't make sense, so make all methods of these classes non-virtual.
(Also delete an unused function on one of the two classes.)

In one case, a class actually did have a subclass that needs to be deleted
virtually, so mark one destructor as virtual.

BUG=none
R=bo_xu@foxitsoftware.com

Review URL: https://codereview.chromium.org/370853002

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