[extensions] Replace getter with readonly property in interface

Bug: none
Change-Id: I3803e8d2ebfd5b98f33ff42530de95331fb3d0b3
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3059611
Auto-Submit: Philip Pfaffe <pfaffe@chromium.org>
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
diff --git a/extension-api/ExtensionAPI.d.ts b/extension-api/ExtensionAPI.d.ts
index 27b1576..b0599ad 100644
--- a/extension-api/ExtensionAPI.d.ts
+++ b/extension-api/ExtensionAPI.d.ts
@@ -9,7 +9,7 @@
     }
 
     export interface Resource {
-      get url(): string;
+      readonly url: string;
 
       getContent(callback: (content: string, encoding: string) => unknown): void;
       setContent(content: string, commit: boolean, callback?: (error?: Object) => unknown): void;