cc: Hoist RasterSource's discard logic up a level

In order to more easily refactor RasterSource's preamble generation
(see https://chromium-review.googlesource.com/c/592237/) this moves
out code that isn't supported by PaintCanvas up to where there's
an SkCanvas.

The SkCanvas::discard logic only applies to gpu raster and so is
only replicated in the gpu raster buffer provider and not elsewhere.

Bug: 671433
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0e006b78bb619b56e5db0a1d30c74e8de63547e1
Reviewed-on: https://chromium-review.googlesource.com/594661
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491202}

cc/raster/gpu_raster_buffer_provider.cc

@@ -76,9 +76,15 @@ static void RasterizeSource(
         100.0f * fraction_saved);
   }
 
+  SkCanvas* canvas = sk_surface->getCanvas();
+
+  // As an optimization, inform Skia to discard when not doing partial raster.
+  if (raster_full_rect == playback_rect)
+    canvas->discard();
+
   raster_source->PlaybackToCanvas(
-      sk_surface->getCanvas(), resource_lock->color_space_for_raster(),
-      raster_full_rect, playback_rect, transform, playback_settings);
+      canvas, resource_lock->color_space_for_raster(), raster_full_rect,
+      playback_rect, transform, playback_settings);
 }
 
 }  // namespace

cc/raster/raster_source.cc

@@ -76,28 +76,9 @@ void RasterSource::PlaybackToCanvas(SkCanvas* input_canvas,
   RasterCommon(raster_canvas, settings.image_provider);
 }
 
-namespace {
-
-bool CanvasIsUnclipped(const SkCanvas* canvas) {
-  if (!canvas->isClipRect())
-    return false;
-
-  SkIRect bounds;
-  if (!canvas->getDeviceClipBounds(&bounds))
-    return false;
-
-  SkISize size = canvas->getBaseLayerSize();
-  return bounds.contains(0, 0, size.width(), size.height());
-}
-
-}  // namespace
-
 void RasterSource::PrepareForPlaybackToCanvas(SkCanvas* canvas) const {
   // TODO(hendrikw): See if we can split this up into separate functions.
 
-  if (CanvasIsUnclipped(canvas))
-    canvas->discard();
-
   // If this raster source has opaque contents, it is guaranteeing that it will
   // draw an opaque rect the size of the layer.  If it is not, then we must
   // clear this canvas ourselves.