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
void BackTexture::Destroy() {
  if (id_ != 0) {
    ScopedGLErrorSuppressor suppressor("BackTexture::Destroy",
                                       state_->GetErrorState());
    glDeleteTextures(1, &id_);
    id_ = 0;
  }
  memory_tracker_.TrackMemFree(bytes_allocated_);
  bytes_allocated_ = 0;
}

void BackTexture::Destroy() {
  if (id_ != 0) {
    ScopedGLErrorSuppressor suppressor("BackTexture::Destroy",
                                       state_->GetErrorState());
    glDeleteTextures(1, &id_);
    id_ = 0;
  }
  memory_tracker_.TrackMemFree(bytes_allocated_);
  bytes_allocated_ = 0;
}
