Steps:
1. Put some things in cache storage
2. Call `session.clearStorageData`

Expected:
Some way of clearing cache storage data

Actual:
No way of clearing cache storage data

This seems like it might just be missing an option, in `atom/browser/api/atom_api_session.cc#GetStorageMask` there are only options for some storages. What's missing is a way of setting any of these:
```
REMOVE_DATA_MASK_CACHE_STORAGE
REMOVE_DATA_MASK_PLUGIN_PRIVATE_DATA
```

* Electron version: 1.8.2
* Operating system: macos 10.13.3

Note: I didn't make a test app for this because it seems like the issue is obvious, let me know if you'd line one though.
atom/browser/api/atom_api_session.cc
atom::VerifyRequestParams val) {
    mate::Dictionary dict = mate::Dictionary::CreateEmpty(isolate);
    dict.Set("hostname", val.hostname);
    dict.Set("certificate", val.certificate);
    dict.Set("verificationResult", val.default_result);
    dict.Set("errorCode", val.error_code);
    return dict.GetHandle();
  }
};
}  
namespace atom {
namespace api {
namespace {
const char kPersistPrefix[] = "persist:";
std::map<uint32_t, v8::Global<v8::Object>> g_sessions;
class ResolveProxyHelper {