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..a1e2e6dc00496731f39ef03b8f19e4cea8bc83e9 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -887,7 +887,7 @@ void DrawingBuffer::paintRenderingResultsToCanvas(ImageBuffer* imageBuffer)
     paintFramebufferToCanvas(framebuffer(), size().width(), size().height(), !m_actualAttributes.premultipliedAlpha, imageBuffer);
 }
 
-PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(int& width, int& height)
+PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(int& width, int& height, SourceBuffer sourceBuffer)
 {
     if (m_actualAttributes.premultipliedAlpha)
         return nullptr;
@@ -903,10 +903,23 @@ PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(in
 
     RefPtr<Uint8ClampedArray> pixels = Uint8ClampedArray::createUninitialized(width * height * 4);
 
-    m_context->bindFramebuffer(GL_FRAMEBUFFER, framebuffer());
+    GLint fbo = 0;
+    if (sourceBuffer == Front && m_frontColorBuffer.texInfo.textureId) {
+        GLint fbo = m_context->createFramebuffer();
+        m_context->bindFramebuffer(GL_FRAMEBUFFER, fbo);
+        m_context->framebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_frontColorBuffer.texInfo.textureId, 0);
+    } else {
+        m_context->bindFramebuffer(GL_FRAMEBUFFER, framebuffer());
+    }
+
     readBackFramebuffer(pixels->data(), width, height, ReadbackRGBA, WebGLImageConversion::AlphaDoNothing);
     flipVertically(pixels->data(), width, height);
 
+    if (fbo) {
+        m_context->framebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 0, 0);
+        m_context->deleteFramebuffer(fbo);
+    }
+
     return pixels.release();
 }