[PaintWorklet] Add some comments for effective_zoom_

It is not obvious why we need to account for effective_zoom_ for the
shadows in the CSS Paint API, but not for line width. This CL adds some
comments to ensure the knowledge won't be lost.

Bug: None
Change-Id: Ibc5086449a929af0d7877874aaec482a2487ab92
Reviewed-on: https://chromium-review.googlesource.com/1090791
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565346}

third_party/blink/renderer/modules/csspaint/paint_rendering_context_2d.cc

@@ -69,6 +69,11 @@ bool PaintRenderingContext2D::ParseColorOrCurrentColor(
   return ::blink::ParseColorOrCurrentColor(color, color_string, nullptr);
 }
 
+// We need to account for the |effective_zoom_| for shadow effects only, and not
+// for line width. This is because the line width is affected by skia's current
+// transform matrix (CTM) while shadows are not. The skia's CTM combines both
+// the canvas context transform and the CSS layout transform. That means, the
+// |effective_zoom_| is implictly applied to line width through CTM.
 double PaintRenderingContext2D::shadowBlur() const {
   return BaseRenderingContext2D::shadowBlur() / effective_zoom_;
 }