Fix potential crash in WebEmbeddedWorkerImpl

Calling Client::workerContextFailedToStart() could immediately delte
WebEmbeddedWorkerImpl, so it shouldn't touch its member variable
after that.

(Without this fix the SW layout test could crash after browser-side
patch lands in chromium)

BUG=285976
TEST=virtual/serviceworker/http/tests/serviceworker/registration.html

Review URL: https://codereview.chromium.org/152883004

git-svn-id: svn://svn.chromium.org/blink/trunk@166731 bbb929c8-8fbe-4397-9dbb-9b2b20218538