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
GLenum Framebuffer::GetColorAttachmentTextureType() const {
  AttachmentMap::const_iterator it = attachments_.find(GL_COLOR_ATTACHMENT0);
  if (it == attachments_.end()) {
    return 0;
  }
  const Attachment* attachment = it->second.get();
  return attachment->texture_type();
}

GLenum Framebuffer::GetColorAttachmentTextureType() const {
  AttachmentMap::const_iterator it = attachments_.find(GL_COLOR_ATTACHMENT0);
  if (it == attachments_.end()) {
    return 0;
  }
  const Attachment* attachment = it->second.get();
  return attachment->texture_type();
}
