Simplify SkPdfTokenLooper behavior.

Simplify SkPdfTokenLooper behavior.

Instead of passing a pointer to a pointer to a NULL looper, which can
then be set to point to a new looper (which then must be looped and
deleted), pass a pointer to the current looper. Each function can then
create a looper based on the parent (on the stack) and call loop()
itself.

Remove setUp(). Now that there is a pointer to the parent at creation
time, there is no need for this function.

Modify the constructors to only provide ones that are needed.

Add documentation.

Remove PdfInlineImageLooper::done(), which is never used.

R=mtklein@google.com

Review URL: https://codereview.chromium.org/83183004

git-svn-id: http://skia.googlecode.com/svn/trunk@12447 2bbb7eff-a529-9590-31e7-b0007b416f81