simplify scrollcanvas to always use shared code

ui/gfx/blit.cc

Index: ui/gfx/blit.cc
diff --git a/ui/gfx/blit.cc b/ui/gfx/blit.cc
index 169eae25aafed3e92e67fe0956d4b61c724e1569..32af55ca3972e7e6c344f7447e4a38fb104222fb 100644
--- a/ui/gfx/blit.cc
+++ b/ui/gfx/blit.cc
@@ -71,25 +71,12 @@ void ScrollCanvas(SkCanvas* canvas,
                   const gfx::Rect& in_clip,
                   const gfx::Vector2d& offset) {
   DCHECK(!HasClipOrTransform(*canvas));  // Don't support special stuff.
-#if defined(OS_WIN)
-  // If we have a PlatformCanvas, we should use ScrollDC. Otherwise, fall
-  // through to the software implementation.
-  if (skia::SupportsPlatformPaint(canvas)) {
-    skia::ScopedPlatformPaint scoped_platform_paint(canvas);
-    HDC hdc = scoped_platform_paint.GetNativeDrawingContext();
-
-    RECT damaged_rect;
-    RECT r = in_clip.ToRECT();
-    ScrollDC(hdc, offset.x(), offset.y(), NULL, &r, NULL, &damaged_rect);
-    return;
-  }
-#endif  // defined(OS_WIN)
-  // For non-windows, always do scrolling in software.
-  // Cairo has no nice scroll function so we do our own. On Mac it's possible to
-  // use platform scroll code, but it's complex so we just use the same path
-  // here. Either way it will be software-only, so it shouldn't matter much.
+
   SkPixmap pixmap;
-  skia::GetWritablePixels(canvas, &pixmap);
+  bool success = skia::GetWritablePixels(canvas, &pixmap);
+  DCHECK(success);

[danakj, 2016/12/21 15:56:11]

Either DCHECK or do the if (!) return, but not both. Can this happen and we need
to handle it or should it never happen?

[reed1, 2016/12/21 15:58:30]

We never checked before, so I'd guess it never happens.

I was following the pattern of DCHECK + return from HasClipOrTransform() just
above. Happy to just do one or the other.

[danakj, 2016/12/21 16:03:29]

Ya that shouldn't be done, our style guide is explicit about that. It looks to
me like DCHECK would be right here looking at the code, but you'd know best.

+  if (!success)
+    return;
 
   // We expect all coords to be inside the canvas, so clip here.
   gfx::Rect clip = gfx::IntersectRects(