SkPDFCanon

SkPDFCanon

SkPDFCanon's fields and methods will eventually become part of
SkPDFDocument/SkDocument_PDF.  For now, it exists as a singleton to
preflight that transition.  This replaces three global arrays in
SkPDFFont, SkPDFShader, and SkPDFGraphicsContext.  This code is still
thread-unsafe (http://skbug.com/2683), but moving this functionality
into SkPDFDocument will fix that.

This CL does not change pdf output from either GMs or SKPs.

This change also simplifies some code around the SkPDFCanon methods.

BUG=skia:2683

Review URL: https://codereview.chromium.org/842253003