Switch SkPDFStream's internal storage from SkStream to SkData

Switch SkPDFStream's internal storage from SkStream to SkData

Motivation: This makes SkPDFStream thread-safe for two threads
serializing it at once, since a SkStream has an internal position.

Updated SkPDFFont, SkPDFGraphicState, and SkPDFPage's use of
SkPDFStream to use the SkData constructor rather than the SkStream
constructor (saving a memcpy).

BUG=skia:2683

Committed: https://skia.googlesource.com/skia/+/c1dfa14b645ae274780f026dd86c9b633fbdad06

R=mtklein@google.com, djsollen@google.com, rmistry@google.com, robertphillips@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/340783013