service worker: Don't crash in context wrapper when context() is null.

PreCreateHostForSharedWorker() was crashing. Stop crashing.

Bug: 931570
Change-Id: I53c4bfcaead20e76a8ca7173ebd5ee3238840fad
Reviewed-on: https://chromium-review.googlesource.com/c/1475134
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632565}
10 files changed