The iOS 8 Release Notes state
The file system layout of app containers has changed on disk. Rather than relying on hard-coded directory structure, use the NSSearchPathForDirectoriesInDomains function or the URLForDirectory:inDomain:appropriateForURL:create:error: method of the NSFileManager class. See Accessing Files and Directories in File System Programming Guide.
This includes the path for [NSBundle mainBundle], which now looks something like
In addition, that UUID part of the path will change when updating the app or installing new builds during development. This means that relying on hard-coded directory structures from legacy code will break not only once, but twice and hard.
For accessing bundled resources from a web view, I think this means that you have to through loading data with – loadData:MIMEType:textEncodingName:baseURL: and supply the mainBundle as the baseURL, instead of just using a request object with the document URL on disk.