gpu: Use a VAO as required by the core profile in ClearFramebuffer

gpu/command_buffer/service/gles2_cmd_clear_framebuffer.cc

Index: gpu/command_buffer/service/gles2_cmd_clear_framebuffer.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_clear_framebuffer.cc b/gpu/command_buffer/service/gles2_cmd_clear_framebuffer.cc
index 6a21c49fd9a05c5f9b8afb82cb66e26a3efa1024..de5e47bb979373c08af7d4e418ec00cc678db5c0 100644
--- a/gpu/command_buffer/service/gles2_cmd_clear_framebuffer.cc
+++ b/gpu/command_buffer/service/gles2_cmd_clear_framebuffer.cc
@@ -73,6 +73,8 @@ void ClearFramebufferResourceManager::Initialize(
       "kVertexPositionAttrib must be 0");
   DCHECK(!buffer_id_);
 
+  glGenVertexArraysOES(1, &vao_);

[Zhenyao Mo, 2016/06/20 21:16:20]

initialize vao_ to 0 in constructor. and dcheck it's 0 above like buffer_id_.

+
   glGenBuffersARB(1, &buffer_id_);
   glBindBuffer(GL_ARRAY_BUFFER, buffer_id_);
   const GLfloat kQuadVertices[] = {-1.0f, -1.0f,
@@ -90,6 +92,7 @@ void ClearFramebufferResourceManager::Destroy() {
     return;
 
   glDeleteProgram(program_);
+  glDeleteVertexArraysOES(1, &vao_);

[Zhenyao Mo, 2016/06/20 21:16:20]

Also set vao_ to 0 like buffer_id_

   glDeleteBuffersARB(1, &buffer_id_);
   buffer_id_ = 0;
 }
@@ -140,7 +143,7 @@ void ClearFramebufferResourceManager::ClearFramebuffer(
     DLOG(ERROR) << "Invalid shader.";
 #endif
 
-  decoder->ClearAllAttributes();
+  glBindVertexArrayOES(vao_);
   glEnableVertexAttribArray(kVertexPositionAttrib);

[Zhenyao Mo, 2016/06/20 21:16:20]

Shouldn't you also set all the vao states up in Initialize, like we did with
buffer_id_, so you don't have to do it in every clear? Simply binding to vao_
will do?

 
   glBindBuffer(GL_ARRAY_BUFFER, buffer_id_);