Fix performance regression in WebGL to 2D canvas draws.

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
index 9ec0e15ee415e2d3c8463837151c3a6201bf026e..2e58bec4cd36bf80e7f4042a0613b8a186f9b851 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
@@ -1126,16 +1126,18 @@ PassRefPtr<Image> HTMLCanvasElement::getSourceImageForCanvas(SourceImageStatus*
         return createTransparentImage(size());
     }
 
+    sk_sp<SkImage> skImage;
     if (m_context->is3d()) {
-        m_context->paintRenderingResultsToCanvas(BackBuffer);
+        // Because WebGL sources always require making a copy of the back buffer, we
+        // use paintRenderingResultsToCanvas instead of getImage in order to keep a cached
+        // copy of the backing in the canvas's ImageBuffer.
+        renderingContext()->paintRenderingResultsToCanvas(BackBuffer);
+        skImage = hasImageBuffer() ? buffer()->newSkImageSnapshot(hint, reason) : createTransparentImage(size())->imageForCurrentFrame();
+    } else {
+        RefPtr<blink::Image> image = renderingContext()->getImage(hint, reason);
+        skImage = image ? image->imageForCurrentFrame() : createTransparentImage(size())->imageForCurrentFrame();
     }
 
-    sk_sp<SkImage> skImage;
-    RefPtr<blink::Image> image = renderingContext()->getImage(reason);
-
-    if (image)
-        skImage = image->imageForCurrentFrame();
-
     if (skImage) {
         *status = NormalSourceImageStatus;
         return StaticBitmapImage::create(std::move(skImage));