Remove a Platform::current() check from Mojo's error handler in WebSocket

The check was added in https://codereview.chromium.org/2258003003/
but is no longer needed because I already removed blink::shutdown
from renderer's shutdown sequence.

BUG=637949

Review-Url: https://codereview.chromium.org/2276433003
Cr-Commit-Position: refs/heads/master@{#415196}
1 file changed