build: update to latest MDC canary and fix error (#19487)

Bumps to the latest canary version of MDC and fixes a build error.

Author: crisbeto

package.json

@@ -59,7 +59,7 @@
     "@types/youtube": "^0.0.38",
     "@webcomponents/custom-elements": "^1.1.0",
     "core-js": "^2.6.9",
-    "material-components-web": "7.0.0-canary.4497b86ed.0",
+    "material-components-web": "7.0.0-canary.8073a20a9.0",
     "rxjs": "^6.5.3",
     "systemjs": "0.19.43",
     "tslib": "^2.0.0",

src/material-experimental/mdc-helpers/BUILD.bazel

@@ -258,8 +258,10 @@ sass_library(
         "@npm//:node_modules/@material/theme/_color-palette.scss",
         "@npm//:node_modules/@material/theme/_constants.import.scss",
         "@npm//:node_modules/@material/theme/_constants.scss",
+        "@npm//:node_modules/@material/theme/_custom-properties.scss",
         "@npm//:node_modules/@material/theme/_functions.import.scss",
         "@npm//:node_modules/@material/theme/_functions.scss",
+        "@npm//:node_modules/@material/theme/_gss.scss",
         "@npm//:node_modules/@material/theme/_index.scss",
         "@npm//:node_modules/@material/theme/_mixins.import.scss",
         "@npm//:node_modules/@material/theme/_mixins.scss",