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

Source/core/html/canvas/WebGLRenderingContextBase.cpp

Index: Source/core/html/canvas/WebGLRenderingContextBase.cpp
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.cpp b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
index 151e37c8648804dee30863e729eda0cf64fff09c..6b9cf49f9f5481824024e3b6a48114020acf8c95 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -91,10 +91,19 @@
 
 namespace blink {
 
+namespace {
+
 const double secondsBetweenRestoreAttempts = 1.0;
 const int maxGLErrorsAllowedToConsole = 256;
 const unsigned maxGLActiveContexts = 16;
 
+DrawingBuffer::SourceBuffer convertSourceBuffer(CanvasRenderingContext::SourceBuffer sourceBuffer)
+{
+    return static_cast<DrawingBuffer::SourceBuffer>(sourceBuffer);
+}
+
+} // namesapce
+
 // FIXME: Oilpan: static vectors to heap allocated WebGLRenderingContextBase objects
 // are kept here. This relies on the WebGLRenderingContextBase finalization to
 // explicitly retire themselves from these vectors, but it'd be preferable if
@@ -915,7 +924,7 @@ void WebGLRenderingContextBase::paintRenderingResultsToCanvas(SourceBuffer sourc
         drawingBuffer()->bind();
 }
 
-PassRefPtrWillBeRawPtr<ImageData> WebGLRenderingContextBase::paintRenderingResultsToImageData()
+PassRefPtrWillBeRawPtr<ImageData> WebGLRenderingContextBase::paintRenderingResultsToImageData(SourceBuffer sourceBuffer)
 {
     if (isContextLost())
         return nullptr;
@@ -923,7 +932,8 @@ PassRefPtrWillBeRawPtr<ImageData> WebGLRenderingContextBase::paintRenderingResul
     clearIfComposited();
     drawingBuffer()->commit();
     int width, height;
-    RefPtr<Uint8ClampedArray> imageDataPixels = drawingBuffer()->paintRenderingResultsToImageData(width, height);
+    RefPtr<Uint8ClampedArray> imageDataPixels =
+        drawingBuffer()->paintRenderingResultsToImageData(width, height, convertSourceBuffer(sourceBuffer));
     if (!imageDataPixels)
         return nullptr;
 
@@ -3557,11 +3567,7 @@ void WebGLRenderingContextBase::texImage2D(GLenum target, GLint level, GLenum in
         }
     }
 
-    RefPtrWillBeRawPtr<ImageData> imageData = canvas->getImageData();
-    if (imageData)
-        texImage2D(target, level, internalformat, format, type, imageData.get(), exceptionState);
-    else
-        texImage2DImpl(target, level, internalformat, format, type, canvas->copiedImage(), WebGLImageConversion::HtmlDomCanvas, m_unpackFlipY, m_unpackPremultiplyAlpha, exceptionState);
+    texImage2DImpl(target, level, internalformat, format, type, canvas->copiedImage(HTMLCanvasElement::Back), WebGLImageConversion::HtmlDomCanvas, m_unpackFlipY, m_unpackPremultiplyAlpha, exceptionState);
 }
 
 PassRefPtr<Image> WebGLRenderingContextBase::videoFrameToImage(HTMLVideoElement* video, BackingStoreCopy backingStoreCopy)
@@ -3795,11 +3801,7 @@ void WebGLRenderingContextBase::texSubImage2D(GLenum target, GLint level, GLint
         || !validateTexFunc("texSubImage2D", TexSubImage2D, SourceHTMLCanvasElement, target, level, format, canvas->width(), canvas->height(), 0, format, type, xoffset, yoffset))
         return;
 
-    RefPtrWillBeRawPtr<ImageData> imageData = canvas->getImageData();
-    if (imageData)
-        texSubImage2D(target, level, xoffset, yoffset, format, type, imageData.get(), exceptionState);
-    else
-        texSubImage2DImpl(target, level, xoffset, yoffset, format, type, canvas->copiedImage(), WebGLImageConversion::HtmlDomCanvas, m_unpackFlipY, m_unpackPremultiplyAlpha, exceptionState);
+    texSubImage2DImpl(target, level, xoffset, yoffset, format, type, canvas->copiedImage(HTMLCanvasElement::Back), WebGLImageConversion::HtmlDomCanvas, m_unpackFlipY, m_unpackPremultiplyAlpha, exceptionState);
 }
 
 void WebGLRenderingContextBase::texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,