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

Source/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 869 matching lines @@
 void DrawingBuffer::setPackAlignment(GLint param)
 {
     m_packAlignment = param;
 }
 
 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;
 
     const SkBitmap& canvasBitmap = imageBuffer->bitmap();
     const SkBitmap* readbackBitmap = 0;
     ASSERT(canvasBitmap.colorType() == kN32_SkColorType);
     if (canvasBitmap.width() == width && canvasBitmap.height() == height) {
         // This is the fastest and most common case. We read back
         // directly into the canvas's backing store.
@@ skipping 98 matching lines @@
 void DrawingBuffer::deleteChromiumImageForTexture(TextureInfo* info)
 {
     if (info->imageId) {
         m_context->releaseTexImage2DCHROMIUM(GL_TEXTURE_2D, info->imageId);
         m_context->destroyImageCHROMIUM(info->imageId);
         info->imageId = 0;
     }
 }
 
 } // namespace blink