Revert of Allow canvases to be GPU-accelerated in Workers

third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp

Index: third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp
diff --git a/third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp b/third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp
index 42347a6eacff6c0757c7c44d98dcdadd03790aa5..96b08881b9914554e5263eda267f112f451c59c1 100644
--- a/third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp
+++ b/third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp
@@ -83,6 +83,8 @@
 
 static bool shouldAccelerate(IntSize surfaceSize)
 {
+    if (!isMainThread())
+        return false; // Add support on Workers crbug.com/
     return RuntimeEnabledFeatures::accelerated2dCanvasEnabled();
 }
 
@@ -117,6 +119,7 @@
     if (!imageBuffer())
         return nullptr;
     sk_sp<SkImage> skImage = m_imageBuffer->newSkImageSnapshot(PreferAcceleration, SnapshotReasonTransferToImageBitmap);
+    DCHECK(isMainThread() || !skImage->isTextureBacked()); // Acceleration not yet supported in Workers
     RefPtr<StaticBitmapImage> image = StaticBitmapImage::create(std::move(skImage));
     image->setOriginClean(this->originClean());
     m_imageBuffer.reset(); // "Transfer" means no retained buffer