cc: Return a NullCanvas instead of NULL in context lost situations.

This reduces the difference in code paths between normal usage and
context lost situations.

Also moves the save()/restore() pair required for GPU raster
into the RasterBuffer class so it can be avoided when unnecessary.

BUG=406404

Review URL: https://codereview.chromium.org/578843002

Cr-Commit-Position: refs/heads/master@{#295290}

cc/resources/resource_provider.cc

@@ -25,6 +25,7 @@
 #include "third_party/khronos/GLES2/gl2ext.h"
 #include "third_party/skia/include/core/SkSurface.h"
 #include "third_party/skia/include/gpu/GrContext.h"
+#include "third_party/skia/include/utils/SkNullCanvas.h"
 #include "ui/gfx/frame_time.h"
 #include "ui/gfx/rect.h"
 #include "ui/gfx/vector2d.h"
@@ -454,8 +455,11 @@ skia::RefPtr<SkCanvas> ResourceProvider::GpuRasterBuffer::AcquireSkCanvas() {
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("cc.debug"),
                "ResourceProvider::GpuRasterBuffer::AcquireSkCanvas");
 
-  return surface_ ? skia::SharePtr(surface_->getCanvas())
-                  : skia::RefPtr<SkCanvas>();
+  skia::RefPtr<SkCanvas> canvas = surface_
+                                      ? skia::SharePtr(surface_->getCanvas())
+                                      : skia::AdoptRef(SkCreateNullCanvas());
+  canvas->save();
+  return canvas;
 }
 
 void ResourceProvider::GpuRasterBuffer::ReleaseSkCanvas(
@@ -463,6 +467,8 @@ void ResourceProvider::GpuRasterBuffer::ReleaseSkCanvas(
   // Note that this function is called from a worker thread.
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("cc.debug"),
                "ResourceProvider::GpuRasterBuffer::ReleaseSkCanvas");
+
+  canvas->restore();
 }
 
 ResourceProvider::ImageRasterBuffer::ImageRasterBuffer(
@@ -506,7 +512,7 @@ skia::RefPtr<SkCanvas> ResourceProvider::ImageRasterBuffer::AcquireSkCanvas() {
                "ResourceProvider::ImageRasterBuffer::AcquireSkCanvas");
 
   if (!mapped_buffer_)
-    return skia::RefPtr<SkCanvas>();
+    return skia::AdoptRef(SkCreateNullCanvas());
 
   MakeBitmap(&raster_bitmap_,
              mapped_buffer_,
@@ -570,7 +576,7 @@ skia::RefPtr<SkCanvas> ResourceProvider::PixelRasterBuffer::AcquireSkCanvas() {
                "ResourceProvider::PixelRasterBuffer::AcquireSkCanvas");
 
   if (!mapped_buffer_)
-    return skia::RefPtr<SkCanvas>();
+    return skia::AdoptRef(SkCreateNullCanvas());
 
   MakeBitmap(&raster_bitmap_,
              mapped_buffer_,

cc/resources/resource_provider_unittest.cc

@@ -3323,9 +3323,8 @@ TEST_P(ResourceProviderTest, PixelBufferLostContext) {
 
   RasterBuffer* raster_buffer = resource_provider->AcquirePixelRasterBuffer(id);
   skia::RefPtr<SkCanvas> canvas = raster_buffer->AcquireSkCanvas();
-  EXPECT_TRUE(canvas.get() == NULL);
+  EXPECT_TRUE(canvas);
   raster_buffer->ReleaseSkCanvas(canvas);
-  canvas.clear();
   resource_provider->ReleasePixelRasterBuffer(id);
   Mock::VerifyAndClearExpectations(context);
 }

cc/resources/tile_manager.cc

@@ -123,9 +123,7 @@ class RasterTaskImpl : public RasterTask {
         devtools_instrumentation::kRasterTask, layer_id_);
 
     skia::RefPtr<SkCanvas> canvas = raster_buffer_->AcquireSkCanvas();
-    if (!canvas)
-      return;
-    canvas->save();
+    DCHECK(canvas);
 
     skia::RefPtr<SkDrawFilter> draw_filter;
     switch (raster_mode_) {
@@ -164,7 +162,6 @@ class RasterTaskImpl : public RasterTask {
           100);
     }
 
-    canvas->restore();
     raster_buffer_->ReleaseSkCanvas(canvas);
   }