Edit comment: remove DisableForAll, which does not exist

Per the comment, the allowable values come from the enum in
feature_policy_features.h. There is no DisableForAll.

TBR=iclelland@chromium.org

Change-Id: I1509a66c570dbb0c2b34876019f965446f91093c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551958Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829566}

third_party/blink/renderer/core/feature_policy/feature_policy_features.json5

@@ -29,7 +29,7 @@
     // public/common/feature_policy/feature_policy_features.h
     feature_default: {
       default: "EnableForSelf",
-      valid_values: ["DisableForAll", "EnableForAll", "EnableForSelf"]
+      valid_values: ["EnableForAll", "EnableForSelf"]
     }
   },