Custom filter table uses removed function

Steps to reproduce

1. Open Advanced tab in options page
2. Add a custom filter "foo"
3. Add a custom filter "bar"
4. Reload page

Observed behavior

• After 2) List contains filter "foo"
• After 3) List contains filter "foo"
• After 4) List contains no filters

Error shown in background page console:

Error in event handler for runtime.onMessage: TypeError: defaultMatcher.isSlowFilter is not a function
    at convertFilter (chrome-extension://gnnljefnoefpefgoofnoicjephgfpamm/lib/adblockplus.js:13110:34)
    at convertSubscription (chrome-extension://gnnljefnoefpefgoofnoicjephgfpamm/lib/adblockplus.js:13094:27)
    at port.on (chrome-extension://gnnljefnoefpefgoofnoicjephgfpamm/lib/adblockplus.js:13500:28)
    at Port._onMessage (chrome-extension://gnnljefnoefpefgoofnoicjephgfpamm/lib/adblockplus.js:4512:22)
    at ext._EventTarget._dispatch (chrome-extension://gnnljefnoefpefgoofnoicjephgfpamm/ext/common.js:41:22)
    at browser.runtime.onMessage.addListener (chrome-extension://gnnljefnoefpefgoofnoicjephgfpamm/ext/background.js:534:26)
    at wrapper (chrome-extension://gnnljefnoefpefgoofnoicjephgfpamm/polyfill.js:152:20)

Expected behavior

• After 2) List contains filter "foo"
• After 3) List contains filters "foo" and "bar"
• After 4) List contains filters "foo" and "bar"

No error shown in background page console.

Further information

The defaultMatcher.isSlowFilter() function was removed in trac#6992 which landed in WebExt in trac#7054.

What to change

Following the integration notes, we need to switch to using the module's global isSlowFilter() function.