WebGL: use storage texture for DrawingBuffer only if CMAA is enabled.

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

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 467 matching lines @@
     m_antiAliasingMode = None;
     if (useMultisampling) {
         m_gl->GetIntegerv(GL_MAX_SAMPLES_ANGLE, &maxSampleCount);
         m_antiAliasingMode = MSAAExplicitResolve;
         if (m_extensionsUtil->supportsExtension("GL_EXT_multisampled_render_to_texture")) {
             m_antiAliasingMode = MSAAImplicitResolve;
         } else if (m_extensionsUtil->supportsExtension("GL_CHROMIUM_screen_space_antialiasing")) {
             m_antiAliasingMode = ScreenSpaceAntialiasing;
         }
     }
-    m_storageTextureSupported = m_webGLVersion > WebGL1 || m_extensionsUtil->supportsExtension("GL_EXT_texture_storage");
+    // TODO(dshwang): enable storage texture on all platform. crbug.com/557848
+    // Linux ATI bot fails WebglConformance.conformance_textures_misc_tex_image_webgl
+    // So use storage texture only if ScreenSpaceAntialiasing is enabled,
+    // because ScreenSpaceAntialiasing is much faster with storage texture.
+    m_storageTextureSupported = (m_webGLVersion > WebGL1
+        ||  m_extensionsUtil->supportsExtension("GL_EXT_texture_storage"))
+        && m_antiAliasingMode == ScreenSpaceAntialiasing;
     m_sampleCount = std::min(4, maxSampleCount);
 
     m_gl->GenFramebuffers(1, &m_fbo);
     m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_fbo);
     if (wantExplicitResolve()) {
         m_gl->GenFramebuffers(1, &m_multisampleFBO);
         m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_multisampleFBO);
         m_gl->GenRenderbuffers(1, &m_multisampleRenderbuffer);
     }
     if (!reset(size))
@@ skipping 617 matching lines @@
 
 void DrawingBuffer::restoreTextureBindings()
 {
     // This class potentially modifies the bindings for GL_TEXTURE_2D and
     // GL_TEXTURE_RECTANGLE. Only GL_TEXTURE_2D needs to be restored since
     // the public interface for WebGL does not support GL_TEXTURE_RECTANGLE.
     m_gl->BindTexture(GL_TEXTURE_2D, m_texture2DBinding);
 }
 
 } // namespace blink