Discard correct drawing buffer

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 85bf211dac13f13d800f3143651e04b9fdbcfba6..05d7e994311e92600367645375d818fc8fd9f692 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -288,6 +288,11 @@ bool DrawingBuffer::prepareMailbox(WebExternalTextureMailbox* outMailbox, WebExt
         if (m_discardFramebufferSupported) {
             // Explicitly discard framebuffer to save GPU memory bandwidth for tile-based GPU arch.
             const GLenum attachments[3] = { GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT };
+            // It appears safe to overwrite the context's framebuffer binding in the Discard case since there will always be a

[Zhenyao Mo, 2016/07/14 17:27:03]

This comment is unnecessary because there is a restoreFramebufferBindings() call
right below.

[qiankun, 2016/07/14 23:40:00]

Thanks for reminding. The comments was added previously when
restoreFramebufferBindings() was not always called.

+            // WebGLRenderingContext::clearIfComposited() call made before the next draw call which restores the framebuffer binding.
+            // If this stops being true at some point, we should track the current framebuffer binding in the DrawingBuffer and restore
+            // it after attaching the new back buffer here.
+            m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_fbo);
             m_gl->DiscardFramebufferEXT(GL_FRAMEBUFFER, 3, attachments);

[qiankun, 2016/07/14 12:37:10]

This only makes framebuffer undefined to save some wasted efforts to load/store
logical buffer (color, depth, stencil) from/to fast memory. See
https://www.khronos.org/registry/gles/extensions/EXT/EXT_discard_framebuffer.txt.
WebGLRenderingContext::clearIfComposited() is still required to do real
clearance.

         }
     } else {