[Dom Storage]: Handle undefined storage case

Description:
There are a ton of moving parts in this code which lends itself to being out of sync at the time of these failures.  Navigation, frame addition and frame removal all trigger a series of separate events that change the main security origin, remove unused origins and add new origins.
The failure is occurs when ClearStorageView._securityOrigin has a mismatch of what origins are in the DOMStorageModel._storages map at the time the user clears the site data.

Issue:
- DomStorageModel.clearForOrigin does not protect against the case where _storages[key] is undefined

Changes:
- Adding !storage check to clearForOrigin
  - we return early because there is no storage to clear and thus a noop is expected
  - also if we resume execution, the following code will attempt to remove the missing storage (will result in no-op) then re-add it (will add a storage that wasn't there previously).

Notes:
- We don't need to worry about the mismatch in the fix since SecurityOriginManager.updateSecurityOrigins will eventually resync the available origins and their storages.

Bug: 1084631
Change-Id: I7bb1abe9182e83239d63478a1c6b68feb7d6451e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2208720
Reviewed-by: Shane Clifford <shanejc@microsoft.com>
Reviewed-by: Robert Paveza <Rob.Paveza@microsoft.com>
Reviewed-by: Vidal Diazleal <vidorteg@microsoft.com>
Commit-Queue: Michael Liao <michael.liao@microsoft.com>

front_end/resources/DOMStorageModel.js

@@ -151,6 +151,9 @@ export class DOMStorageModel extends SDK.SDKModel.SDKModel {
     for (const isLocal of [true, false]) {
       const key = this._storageKey(origin, isLocal);
       const storage = this._storages[key];
+      if (!storage) {
+        return;
+      }
       storage.clear();
     }
     this._removeOrigin(origin);