M35 Gardening: Do not initialize XWalkRunner twice

This is all being created already when we create a XWalkMainDelegate
at XWalkTestLauncherDelegate.

Reflects the simplification landed here:
https://codereview.chromium.org/190663012
Merge pull request #1770 from tmpsantos/next

M35 Gardening: Do not initialize XWalkRunner twice