SkPDF: speed up SkPDFShader generation.

Stop using SkString::append() when SkDynamicMemoryWStream
works better.

Also add a bench to prove that this speeds things up:
    before:
        micros   	bench
         59.33 ?	PDFShader	nonrendering
    after:
        micros   	bench
         34.55 ?	PDFShader	nonrendering
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Review URL: https://codereview.chromium.org/1833793002
3 files changed