[PE] Distinguish between tainting due to canvas content and filter.

A filter on a canvas can itself lead to origin tainting, for reasons
other than that the canvas contents are tainted. This CL changes
to distinguish these two causes, so that we recompute filters
on content-tainting change.

Bug: 778506
Change-Id: I3cec8ef3b2772f2af78cdd4b290520113092cca6
Reviewed-on: https://chromium-review.googlesource.com/811767
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522274}
bool BaseRenderingContext2D::IsFullCanvasCompositeMode(SkBlendMode op) {
  return op == SkBlendMode::kSrcIn || op == SkBlendMode::kSrcOut ||
         op == SkBlendMode::kDstIn || op == SkBlendMode::kDstATop;
}

bool BaseRenderingContext2D::IsFullCanvasCompositeMode(SkBlendMode op) {
  return op == SkBlendMode::kSrcIn || op == SkBlendMode::kSrcOut ||
         op == SkBlendMode::kDstIn || op == SkBlendMode::kDstATop;
}
