OOP: Ensure to clear tiles when needed even if no display items.

This patch ensures that we don't early out under requires_clear when
doing OOP raster. If we early out, and we were doing a partial raster,
then we might end up with previous content on the texture.

R=enne@chromium.org, piman@chromium.org

Bug: 901897
Change-Id: I9e74e7f5b04d14385b733df50b08e027f5e6d1d3
Reviewed-on: https://chromium-review.googlesource.com/c/1335662
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608391}

cc/paint/oop_pixeltest.cc

@@ -1209,8 +1209,11 @@ TEST_F(OopPixelTest, ClearingTransparentInternalTile) {
   options.preclear = true;
   options.preclear_color = SK_ColorRED;
 
-  // Make a non-empty but noop display list to avoid early outs.
-  auto display_item_list = MakeNoopDisplayItemList();
+  // Note that clearing of the tile should supersede any early outs due to an
+  // empty display list. This is due to the fact that partial raster may in fact
+  // result in no items being generated, in which case a clear should still
+  // happen. See crbug.com/901897.
+  auto display_item_list = base::MakeRefCounted<DisplayItemList>();
 
   auto oop_result = Raster(display_item_list, options);
   auto gpu_result = RasterExpectedBitmap(display_item_list, options);

gpu/command_buffer/client/raster_implementation.cc

@@ -1263,7 +1263,11 @@ void RasterImplementation::RasterCHROMIUM(const cc::DisplayItemList* list,
   gfx::Rect query_rect =
       gfx::ScaleToEnclosingRect(playback_rect, 1.f / post_scale);
   list->rtree_.Search(query_rect, &temp_raster_offsets_);
-  if (temp_raster_offsets_.empty())
+  // We can early out if we have nothing to draw and we don't need a clear. Note
+  // that if there is nothing to draw, but a clear is required, then those
+  // commands would be serialized in the preamble and it's important to play
+  // those back.
+  if (temp_raster_offsets_.empty() && !requires_clear)
     return;
 
   // TODO(enne): Tune these numbers