gpu: Clarify sized texture format is available only if ES3 context or immutable texture is supported

gpu/command_buffer/service/gles2_cmd_decoder.cc

 // Copyright (c) 2012 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_decoder.h"
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
@@ skipping 16123 matching lines @@
                    depth);
   }
   if (workarounds().reset_base_mipmap_level_before_texstorage &&
       texture->base_level() > 0)
     glTexParameteri(target, GL_TEXTURE_BASE_LEVEL, texture->base_level());
 
   {
     GLsizei level_width = width;
     GLsizei level_height = height;
     GLsizei level_depth = depth;
-    GLenum adjusted_format =
-        feature_info_->IsES3Enabled() ? internal_format : format;

[dshwang, 2016/10/10 20:57:10]

Here is inside GLES2DecoderImpl::TexStorageImpl().

texture_manager()->SetLevelInfo() checks if the sized format is registered in
|texture_sized_xxx_internal_format|

     for (int ii = 0; ii < levels; ++ii) {
       if (target == GL_TEXTURE_CUBE_MAP) {
         for (int jj = 0; jj < 6; ++jj) {
           GLenum face = GL_TEXTURE_CUBE_MAP_POSITIVE_X + jj;
-          texture_manager()->SetLevelInfo(texture_ref, face, ii,
-                                          adjusted_format,
-                                          level_width, level_height, 1,
-                                          0, format, type, gfx::Rect());
+          texture_manager()->SetLevelInfo(
+              texture_ref, face, ii, internal_format, level_width, level_height,
+              1, 0, format, type, gfx::Rect());
         }
       } else {
-        texture_manager()->SetLevelInfo(texture_ref, target, ii,
-                                        adjusted_format,
-                                        level_width, level_height, level_depth,
-                                        0, format, type, gfx::Rect());
+        texture_manager()->SetLevelInfo(
+            texture_ref, target, ii, internal_format, level_width, level_height,
+            level_depth, 0, format, type, gfx::Rect());
       }
       level_width = std::max(1, level_width >> 1);
       level_height = std::max(1, level_height >> 1);
       if (target == GL_TEXTURE_3D)
         level_depth = std::max(1, level_depth >> 1);
     }
     texture->SetImmutable(true);
   }
 }
 
@@ skipping 2077 matching lines @@
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "base/macros.h"
 #include "gpu/command_buffer/service/gles2_cmd_decoder_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu