Revert "weblayer, skew tests: Remove goma mixin from mb_config.pyl used to"

The GOMA freeze has ended. Now we can use GOMA to compile the weblayer
skew tests on the refresh-weblayer-skew-tests builder.

Bug: 1041619, 1099017
Change-Id: I0794c836add2498b8dfc9f52f3b82ff7fbd4ff6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616959Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Rakib Hasan <rmhasan@google.com>
Commit-Queue: Rakib Hasan <rmhasan@google.com>
Cr-Commit-Position: refs/heads/master@{#841326}

weblayer/browser/android/javatests/skew/mb_config.pyl

@@ -67,7 +67,7 @@
     },
 
     'release_bot': {
-      'mixins': ['release', 'static'],
+      'mixins': ['release', 'static', 'goma'],
     },
 
     'static': {