SkPDF: cache metrics once.

SkPDF:  cache metrics once.

Motivation: drawText can look up unicode mapping at draw time to see
if ActualText should be used after crrev.com/2084533004 lands.

For each SkTypeface, only call getAdvancedTypefaceMetrics() once per
document.  Cache the result in the SkPDFCanon, indexed by SkFontID.
Also cache PDF FontDescriptors in the canon, also indexed by SkFontID
(Type1 fonts only).

Simplify PDF font lookup, map SkFontID+SkGlyphID into a uint64_t.  Map
that uint64_t to SkPDFFonts.  Remove SkPDFCanon::findFont(),
SkPDFCanon::addFont(), SkPDFFont::IsMatch(), and enum SkPDFFont::Match.

SkPDFFont no longer holds on to ref of SkAdvancedTypefaceMetrics.
Instead, SkPDFFont::GetFontResource() and SkPDFFont::getFontSubset()
get metrics from canon.

SkPDFFont multybite bool is now a function of type.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2253993002

Review-Url: https://codereview.chromium.org/2253993002