Auto merge of #29637 - AnthonyJ3:Use-same-origin-as-default-credential-mode-for-RequestInit-fix, r=mukilan

Use same-origin as default credential mode for RequestInit fix

<!-- Please describe your changes on the following line: -->
- Updated the default credential mode from omit to same-origin as per: https://github.com/whatwg/fetch/pull/1153
- Deleted instances of fallback credentials
---

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #29633  (GitHub issue number if applicable)
- [X] There are tests for these changes

components/net_traits/request.rs

@@ -270,7 +270,7 @@ impl RequestBuilder {
             mode: RequestMode::NoCors,
             cache_mode: CacheMode::Default,
             use_cors_preflight: false,
-            credentials_mode: CredentialsMode::Omit,
+            credentials_mode: CredentialsMode::CredentialsSameOrigin,
             use_url_credentials: false,
             origin: ImmutableOrigin::new_opaque(),
             referrer: referrer,
@@ -517,7 +517,7 @@ impl Request {
             synchronous: false,
             mode: RequestMode::NoCors,
             use_cors_preflight: false,
-            credentials_mode: CredentialsMode::Omit,
+            credentials_mode: CredentialsMode::CredentialsSameOrigin,
             use_url_credentials: false,
             cache_mode: CacheMode::Default,
             redirect_mode: RedirectMode::Follow,