WebGL: clarify which Front or Back buffer is used by each API.

Source/platform/graphics/ImageBuffer.cpp

Index: Source/platform/graphics/ImageBuffer.cpp
diff --git a/Source/platform/graphics/ImageBuffer.cpp b/Source/platform/graphics/ImageBuffer.cpp
index c7ac9b2c6be857e29180f1faa077f4ce4cd2073a..89d59aa3c86fdb6c6e4f6e4e0756701de2df8e10 100644
--- a/Source/platform/graphics/ImageBuffer.cpp
+++ b/Source/platform/graphics/ImageBuffer.cpp
@@ -435,15 +435,4 @@ String ImageBuffer::toDataURL(const String& mimeType, const double* quality) con
     return "data:" + mimeType + ";base64," + base64Encode(encodedImage);
 }
 
-String ImageDataToDataURL(const ImageDataBuffer& imageData, const String& mimeType, const double* quality)
-{
-    ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
-
-    Vector<char> encodedImage;
-    if (!encodeImage(imageData, mimeType, quality, &encodedImage))
-        return "data:,";
-
-    return "data:" + mimeType + ";base64," + base64Encode(encodedImage);
-}
-
 } // namespace blink