Re-land "WebGL: Transfer ownership of WebGraphicsContext3D to DrawingBuffer"

Source/core/html/canvas/WebGLDrawBuffers.cpp

Index: Source/core/html/canvas/WebGLDrawBuffers.cpp
diff --git a/Source/core/html/canvas/WebGLDrawBuffers.cpp b/Source/core/html/canvas/WebGLDrawBuffers.cpp
index c85a98828ea9e3460fcb1422b401b6c60ea4e47d..e5b6b48659f43cc87c810df8b7cd5aad19b7c4de 100644
--- a/Source/core/html/canvas/WebGLDrawBuffers.cpp
+++ b/Source/core/html/canvas/WebGLDrawBuffers.cpp
@@ -79,7 +79,7 @@ void WebGLDrawBuffers::drawBuffersWEBGL(const Vector<GLenum>& buffers)
         }
         // Because the backbuffer is simulated on all current WebKit ports, we need to change BACK to COLOR_ATTACHMENT0.
         GLenum value = (bufs[0] == GL_BACK) ? GL_COLOR_ATTACHMENT0 : GL_NONE;
-        m_context->webGraphicsContext3D()->drawBuffersEXT(1, &value);
+        m_context->webContext()->drawBuffersEXT(1, &value);
         m_context->setBackDrawBuffer(bufs[0]);
     } else {
         if (n > m_context->maxDrawBuffers()) {
@@ -99,7 +99,7 @@ void WebGLDrawBuffers::drawBuffersWEBGL(const Vector<GLenum>& buffers)
 // static
 bool WebGLDrawBuffers::satisfiesWebGLRequirements(WebGLRenderingContextBase* webglContext)
 {
-    blink::WebGraphicsContext3D* context = webglContext->webGraphicsContext3D();
+    blink::WebGraphicsContext3D* context = webglContext->webContext();
     Extensions3DUtil* extensionsUtil = webglContext->extensionsUtil();
 
     // This is called after we make sure GL_EXT_draw_buffers is supported.