SkPDF: SkPDFGraphicState Lookup hashtabled

In Release, running `dm --src skp --config pdf`, I get a
speedup of about 1.2%.

SkPDFGraphicState class:
-   Holds the subset of SkPaint that maps to a PDF Graphics
    State
-   These fields are easily comparable, making hashtable
    comparisons easy.

SkPDFCanon:
-   findGraphicState() takes a SkPDFGraphicState, not a SkPaint
-   fGraphicStateRecords is a SkHashSet, not a SkTDArray

SkPDFGraphicState:
-   mode_for_pdf() replaces logic inside equivalent(), but is
    only called once per lookup.
-   emitObject() no longer modifies the SkPDFGraphicState to
    cache the SkPDFDict stucture.  (Since it is de-duped,
    this get no speedup).
-   Static Functions that don't use the canon return a plain
    SkPDFDict now.  No need for fPopulated.

SkTHash.h
-   SkHashSet::forall added

SkPDFDevice; SkPDFShader
-   Updated for new SkPDFGraphicState interface.

BUG=skia:3585

Review URL: https://codereview.chromium.org/1046293002

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