ServiceWorker: Remove unused startMode field from WebEmbeddedWorkerStartData [2/2]

[1] Chromium-side: https://codereview.chromium.org/688803002/
[2] Blink-side: THIS PATCH

BUG=n/a
TEST=compile

Review URL: https://codereview.chromium.org/686353003

git-svn-id: svn://svn.chromium.org/blink/trunk@184685 bbb929c8-8fbe-4397-9dbb-9b2b20218538