Quota: Remove RuntimeEnabledFlag for quota usage details.

The feature gated by StorageQuotaDetails has launched in Chrome 75. At
this point, it's safe to assume the feature will remain launched.

https://chromestatus.com/feature/5737131066523648

Change-Id: I3aa6b14a5b53ef07df73c43b923f1c77d89dcdb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072822
Commit-Queue: Victor Costan <pwnall@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744591}

third_party/blink/renderer/modules/quota/storage_estimate.idl

@@ -7,5 +7,5 @@
 dictionary StorageEstimate {
     unsigned long long usage;
     unsigned long long quota;
-    [RuntimeEnabled=StorageQuotaDetails] StorageUsageDetails usageDetails;
+    StorageUsageDetails usageDetails;
 };

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1632,10 +1632,6 @@
       name: "StorageAccessAPI",
       status: "test",
     },
-    {
-      name: "StorageQuotaDetails",
-      status: "stable"
-    },
     {
       name: "StrictMimeTypesForWorkers",
       status: "experimental"