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

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 60a9d9862c845d8daf7e086ded9f085bd0552242..9512fd03415b67777954efc24cb2d769155bb8c5 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -887,29 +887,6 @@ void DrawingBuffer::paintRenderingResultsToCanvas(ImageBuffer* imageBuffer)
     paintFramebufferToCanvas(framebuffer(), size().width(), size().height(), !m_actualAttributes.premultipliedAlpha, imageBuffer);
 }
 
-PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(int& width, int& height)
-{
-    if (m_actualAttributes.premultipliedAlpha)
-        return nullptr;
-
-    width = size().width();
-    height = size().height();
-
-    Checked<int, RecordOverflow> dataSize = 4;
-    dataSize *= width;
-    dataSize *= height;
-    if (dataSize.hasOverflowed())
-        return nullptr;
-
-    RefPtr<Uint8ClampedArray> pixels = Uint8ClampedArray::createUninitialized(width * height * 4);
-
-    m_context->bindFramebuffer(GL_FRAMEBUFFER, framebuffer());
-    readBackFramebuffer(pixels->data(), width, height, ReadbackRGBA, WebGLImageConversion::AlphaDoNothing);
-    flipVertically(pixels->data(), width, height);
-
-    return pixels.release();
-}
-
 void DrawingBuffer::paintFramebufferToCanvas(int framebuffer, int width, int height, bool premultiplyAlpha, ImageBuffer* imageBuffer)
 {
     unsigned char* pixels = 0;