Chore: Add placeholder for internal import of deprecated styles. (#6419)

Googlers, see cl/537831311, which requires a placeholder be added to the
code to _variable.scss in order for a new import transform to be
applied.

Author: bmd3k

tensorboard/webapp/theme/_variable.scss

@@ -12,6 +12,7 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 ==============================================================================*/
+// INTERNAL-ONLY: Import of deprecated legacy Angular Material styles
 @use '@angular/material' as mat;
 @import 'tensorboard/webapp/theme/tb_palette';