cc: Avoid extra flush calls to Skia GrContext

cc/raster/gpu_rasterizer.cc

Index: cc/raster/gpu_rasterizer.cc
diff --git a/cc/raster/gpu_rasterizer.cc b/cc/raster/gpu_rasterizer.cc
index 9525d8f9f2220e8e5085b17f9ddb7d30bb848f29..8beaccd11ab7fadcc923a026731a3aaef5515e20 100644
--- a/cc/raster/gpu_rasterizer.cc
+++ b/cc/raster/gpu_rasterizer.cc
@@ -79,7 +79,16 @@ void GpuRasterizer::RasterizeSource(
 
     SkMultiPictureDraw multi_picture_draw;
     multi_picture_draw.add(sk_surface->getCanvas(), picture.get());
-    multi_picture_draw.draw(msaa_sample_count_ > 0);
+    // This is undocumented:

[bsalomon, 2016/02/01 14:55:59]

What is undocumented? Are we missing something on the Skia side?

+    // SkCanvas::flush must be explicitly called in order to flush possibly
+    // deferred rendering in SkGpuDevice.
+    // This also prepares the GrRenderTarget of the sk_surface for external
+    // read.
+    // Correct point of preparing would be in
+    // ScopedWriteLockGr::ReleaseSkSurface, but this would be redundant, as
+    // SkGpuDevice::flush forces it.
+    // Passing true SkMultiPictureDraw::draw(bool flush) makes this happen.
+    multi_picture_draw.draw(true);
     write_lock->ReleaseSkSurface();
   }
 }