[PaintWorklet] Do not paint when paint target is associated with a link

When the target element of a paint worklet has an associated link, then
the 'paint' function will be invoked when the link's href is changed
from a visited URL to an unvisited URL (or vice versa).

This CL changes the behavior by detecting whether the target element
of a paint worklet has an associated link or not. If it does, then don't
paint.

TBR=haraken@chromium.org

Bug: 835589
Change-Id: I5fdf85685f863c960a6f48cc9a345dda787bece1
Reviewed-on: https://chromium-review.googlesource.com/1035524
Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555788}
bool CSSPaintValue::KnownToBeOpaque(const Document&,
                                    const ComputedStyle&) const {
  return generator_ && !generator_->HasAlpha();
}

bool CSSPaintValue::KnownToBeOpaque(const Document&,
                                    const ComputedStyle&) const {
  return generator_ && !generator_->HasAlpha();
}
