Re-enable GPU partial raster for non-MSAA content

This re-enables GPU partial raster for non-MSAA content. MSAA resolve
does not work well with partial raster right now - we may try to address
this in the future.

BUG=629683
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2161933004
Cr-Commit-Position: refs/heads/master@{#406642}

cc/raster/gpu_raster_buffer_provider.cc

@@ -226,7 +226,10 @@ bool GpuRasterBufferProvider::IsResourceSwizzleRequired(
 }
 
 bool GpuRasterBufferProvider::CanPartialRasterIntoProvidedResource() const {
-  return false;
+  // Partial raster doesn't support MSAA, as the MSAA resolve is unaware of clip
+  // rects.
+  // TODO(crbug.com/629683): See if we can work around this limitation.
+  return msaa_sample_count_ == 0;
 }
 
 void GpuRasterBufferProvider::Shutdown() {

cc/trees/layer_tree_host_pixeltest_tiles.cc

@@ -223,10 +223,8 @@ TEST_F(LayerTreeHostTilesTestPartialInvalidation,
       base::FilePath(FILE_PATH_LITERAL("blue_yellow_flipped.png")));
 }
 
-// TODO(crbug.com/629683): Re-enable this test once GPU partial raster issues
-// are fixed.
 TEST_F(LayerTreeHostTilesTestPartialInvalidation,
-       DISABLED_PartialRaster_SingleThread_GpuRaster) {
+       PartialRaster_SingleThread_GpuRaster) {
   RunRasterPixelTest(
       false, PARTIAL_GPU, picture_layer_,
       base::FilePath(FILE_PATH_LITERAL("blue_yellow_partial_flipped.png")));