Simplify GraphicsContext::ApplyHighContrastFilter implementations

We assume that SkColor <=> Color::Rgb(), so simplify the Color variant
accordingly.

PaintFlags are always passed as constant references, so pass it like
that to the PaintFlags variant too and remove the null-checks. Also
always explicitly copy the PaintFlags so that we get RVO.

Change-Id: I46b6f5aff7cbe175d6b9a2ff8277ff5ce9b4fce8
Reviewed-on: https://chromium-review.googlesource.com/c/1358497
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#613158}
2 files changed