SkPDF: skip shader lookup for SkShader::kColor_GradientType

Also: SkPDFShader::State isi now zero-initilized.

No change in PDF tests.

BUG=chromium:690875
Change-Id: Ibc56cc9435362733adf50cbb51b11c9413572e7f
Reviewed-on: https://skia-review.googlesource.com/8355
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Hal Canary <halcanary@google.com>
2 files changed