SkPDF:  alloc less memory for strings

    before:
        micros   	bench
        250.98  	WritePDFText	nonrendering
    after:
        micros   	bench
        107.10  	WritePDFText	nonrendering

Also, be slightly more space-efficient in encoding strings.

Also, add a bench.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2099463002

Review-Url: https://codereview.chromium.org/2099463002
6 files changed