iOS8WKWebview没有提供清理缓存的接口，但是可以用下面的方法来清理

- (void)clearWebCache {
    if ([[UIDevice currentDevice].systemVersion floatValue] >= 9.0) {
        
        NSSet *websiteDataTypes
        
        = [NSSet setWithArray:@[
                                WKWebsiteDataTypeDiskCache,
                                
                                //WKWebsiteDataTypeOfflineWebApplicationCache,
                                
                                WKWebsiteDataTypeMemoryCache,
                                
                                //WKWebsiteDataTypeLocalStorage,
                                
                                //WKWebsiteDataTypeCookies,
                                
                                //WKWebsiteDataTypeSessionStorage,
                                
                                //WKWebsiteDataTypeIndexedDBDatabases,
                                
                                //WKWebsiteDataTypeWebSQLDatabases
                                ]];
        
        //// All kinds of data
        
        //NSSet *websiteDataTypes = [WKWebsiteDataStore allWebsiteDataTypes];
        
        //// Date from
        
        NSDate *dateFrom = [NSDate dateWithTimeIntervalSince1970:0];
        
        //// Execute
        
        [[WKWebsiteDataStore defaultDataStore] removeDataOfTypes:websiteDataTypes modifiedSince:dateFrom completionHandler:^{
            
            // Done
            
        }];
    } else {
        NSString *libraryPath = [NSSearchPathForDirectoriesInDomains(NSLibraryDirectory, NSUserDomainMask, YES) objectAtIndex:0];
        
        NSString *cookiesFolderPath = [libraryPath stringByAppendingString:@"/Cookies"];
        
        NSError *errors;
        
        [[NSFileManager defaultManager] removeItemAtPath:cookiesFolderPath error:&errors];
    }
}