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

gpu/command_buffer/service/gles2_cmd_clear_framebuffer.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "gpu/command_buffer/service/gles2_cmd_clear_framebuffer.h"
 
 #include "gpu/command_buffer/service/gl_utils.h"
 #include "gpu/command_buffer/service/gles2_cmd_decoder.h"
 #include "ui/gfx/geometry/size.h"
 
@@ skipping 55 matching lines @@
   DCHECK(!buffer_id_);
 }
 
 void ClearFramebufferResourceManager::Initialize(
     const gles2::GLES2Decoder* decoder) {
   static_assert(
       kVertexPositionAttrib == 0u,
       "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,
                                     1.0f, -1.0f,
                                     1.0f,  1.0f,
                                    -1.0f,  1.0f};
   glBufferData(
       GL_ARRAY_BUFFER, sizeof(kQuadVertices), kQuadVertices, GL_STATIC_DRAW);
   decoder->RestoreBufferBindings();
   initialized_ = true;
 }
 
 void ClearFramebufferResourceManager::Destroy() {
   if (!initialized_)
     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;
 }
 
 void ClearFramebufferResourceManager::ClearFramebuffer(
     const gles2::GLES2Decoder* decoder,
     const gfx::Size& framebuffer_size,
     GLbitfield mask,
     GLfloat clear_color_red,
     GLfloat clear_color_green,
@@ skipping 30 matching lines @@
   glUseProgram(program_);
 
 #if DCHECK_IS_ON()
   glValidateProgram(program_);
   GLint validation_status = GL_FALSE;
   glGetProgramiv(program_, GL_VALIDATE_STATUS, &validation_status);
   if (GL_TRUE != validation_status)
     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_);
   glVertexAttribPointer(kVertexPositionAttrib, 2, GL_FLOAT, GL_FALSE, 0, 0);
 
   glUniform1f(depth_handle_, clear_depth_value);
   glUniform4f(color_handle_, clear_color_red, clear_color_green,
               clear_color_blue, clear_color_alpha);
 
   if (!(mask & GL_COLOR_BUFFER_BIT)) {
     glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
@@ skipping 17 matching lines @@
     glStencilMask(0);
   }
 
   glDisable(GL_CULL_FACE);
   glDisable(GL_BLEND);
   glDisable(GL_POLYGON_OFFSET_FILL);
 
   glViewport(0, 0, framebuffer_size.width(), framebuffer_size.height());
   glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
 
   decoder->RestoreAllAttributes();

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

This seems like an over kill. You just need to restore the vao bindings here,
nothing else.

   decoder->RestoreProgramBindings();
   decoder->RestoreBufferBindings();
   decoder->RestoreGlobalState();
 }
 
 }  // namespace gpu