cc: Add some class level comments for PaintCanvas

Change-Id: Ib652cc3653bf278c2cb9267cad12d1ee93f63683
Reviewed-on: https://chromium-review.googlesource.com/c/1292725Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602048}

cc/paint/paint_canvas.h

@@ -21,6 +21,19 @@ class PaintOpBuffer;
 
 using PaintRecord = PaintOpBuffer;
 
+// PaintCanvas is the cc/paint wrapper of SkCanvas.  It has a more restricted
+// interface than SkCanvas (trimmed back to only what Chrome uses).  Its reason
+// for existence is so that it can do custom serialization logic into a
+// PaintOpBuffer which (unlike SkPicture) is mutable, handles image replacement,
+// and can be serialized in custom ways (such as using the transfer cache).
+//
+// PaintCanvas is usually implemented by either:
+// (1) SkiaPaintCanvas, which is backed by an SkCanvas, usually for rasterizing.
+// (2) RecordPaintCanvas, which records paint commands into a PaintOpBuffer.
+//
+// SkiaPaintCanvas allows callers to go from PaintCanvas to SkCanvas (or
+// PaintRecord to SkPicture), but this is a one way trip.  There is no way to go
+// from SkCanvas to PaintCanvas or from SkPicture back into PaintRecord.
 class CC_PAINT_EXPORT PaintCanvas {
  public:
   PaintCanvas() {}

cc/paint/record_paint_canvas.cc

@@ -315,7 +315,7 @@ SkNoDrawCanvas* RecordPaintCanvas::GetCanvas() {
     return &*canvas_;
 
   // Size the canvas to be large enough to contain the |recording_bounds|, which
-  // may not be positioned at th origin.
+  // may not be positioned at the origin.
   SkIRect enclosing_rect = recording_bounds_.roundOut();
   canvas_.emplace(enclosing_rect.right(), enclosing_rect.bottom());