SkPDF: Glyph Useage Map improvements

SkPDF: Glyph Useage Map improvements

Instead of having a fFontGlyphUsage on each device and one on each
document, just have the one on the document, and never merge.

Make fGlyphUsage accesible on SkPDFDocument.

Remove SkPDFGlyphSetMap::merge, ::reset, and SkPDFGlyphSet::merge.

SkPDFGlyphSetMap has an TArray of SkPDFGlyphSet, not TDArray of
SkPDFGlyphSet pointers.  SkPDFGlyphSet and SkPDFBitset get move
constructors.

All tests produce exactly identical output PDFs.

BUG=skia:5434
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2112943002

Review-Url: https://codereview.chromium.org/2112943002