Framebuffer clear() needs to consider the situation some draw buffers are disabled.

This is when we expose DrawBuffers extension.

BUG=376951
TEST=the attached test case, webgl conformance
R=kbr@chromium.org,bajones@chromium.org

Review URL: https://codereview.chromium.org/315283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275338 0039d316-1c4b-4281-b951-d872f2087c98
bool GLES2DecoderImpl::GetServiceTextureId(uint32 client_texture_id,
                                           uint32* service_texture_id) {
  TextureRef* texture_ref = texture_manager()->GetTexture(client_texture_id);
  if (texture_ref) {
    *service_texture_id = texture_ref->service_id();
    return true;
  }
  return false;
}

bool GLES2DecoderImpl::GetServiceTextureId(uint32 client_texture_id,
                                           uint32* service_texture_id) {
  TextureRef* texture_ref = texture_manager()->GetTexture(client_texture_id);
  if (texture_ref) {
    *service_texture_id = texture_ref->service_id();
    return true;
  }
  return false;
}
