SkPDF: Always get advances at unitsPerEm.

SkPDF: Always get advances at unitsPerEm.

  * Work around BUG=chromium:696356
  * SkTestScalerContext needs a return a em-size.
  * SkPDFFont::MakeVectorCache which always produces a glyph
    cache at emsize.  Replaces vector_cache().
  * Stop looking at fLastGlyphID and fEmSize in TypefaceMetrics.

Change-Id: I28d93b8f62d461a60fa046e9aaf7fa6d116a7ee5
Reviewed-on: https://skia-review.googlesource.com/9324
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>