WebGL: paintRenderingResultsToImageData() must restore FBO.

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 73509401a7523f0d54621d315c75bdc456762412..d7a0c97a513a752d6ca4c9a569108a8386cb91aa 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -903,9 +903,7 @@ void DrawingBuffer::paintRenderingResultsToCanvas(ImageBuffer* imageBuffer)
 
 bool DrawingBuffer::paintRenderingResultsToImageData(int& width, int& height, SourceDrawingBuffer sourceBuffer, WTF::ArrayBufferContents& contents)
 {
-    if (m_actualAttributes.premultipliedAlpha)
-        return false;
-
+    ASSERT(!m_actualAttributes.premultipliedAlpha);
     width = size().width();
     height = size().height();