Deal gracefully with null {GraphicsContext,SkPictureBuilder}.endRecording() results.

third_party/WebKit/Source/platform/graphics/paint/SkPictureBuilder.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SkPictureBuilder_h
 #define SkPictureBuilder_h
 
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/paint/PaintController.h"
 #include "wtf/Allocator.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/OwnPtr.h"
 
 namespace blink {
 
 // When slimming paint ships we can remove this SkPicture abstraction and
 // rely on PaintController here.
 class SkPictureBuilder final {
     WTF_MAKE_NONCOPYABLE(SkPictureBuilder);
 public:
+    // Constructs a new builder with the given bounds for the resulting recorded picture. If
+    // |metadata| is specified, that metadata is propagated to the builder's internal canvas. If
+    // |containingContext| is specified, the device scale factor, printing, and disabled state are
+    // propagated to the builder's internal context.
     SkPictureBuilder(const FloatRect& bounds, SkMetaData* metaData = nullptr, GraphicsContext* containingContext = nullptr)
         : m_bounds(bounds)
     {
         GraphicsContext::DisabledMode disabledMode = GraphicsContext::NothingDisabled;
         if (containingContext && containingContext->contextDisabled())
             disabledMode = GraphicsContext::FullyDisabled;
 
         m_paintController = PaintController::create();
         m_paintController->beginSkippingCache();
         m_context = adoptPtr(new GraphicsContext(*m_paintController, disabledMode, metaData));
 
         if (containingContext) {
             m_context->setDeviceScaleFactor(containingContext->deviceScaleFactor());
             m_context->setPrinting(containingContext->printing());
         }
     }
 
     GraphicsContext& context() { return *m_context; }
 
+    // Returns a picture capturing all drawing performed on the builder's context since
+    // construction, or nullptr if the picture could not be created.
     PassRefPtr<SkPicture> endRecording()
     {
         m_context->beginRecording(m_bounds);
         m_paintController->endSkippingCache();
         m_paintController->commitNewDisplayItems();
         m_paintController->paintArtifact().replay(*m_context);
         return m_context->endRecording();
     }
 
 private:
     OwnPtr<PaintController> m_paintController;
     OwnPtr<GraphicsContext> m_context;
     FloatRect m_bounds;
 };
 
 } // namespace blink
 
 #endif // SkPictureBuilder_h