Cleanup: Pass worker startup parameters as a single WorkerThreadStartupData struct

WorkerGlobalScope and WorkerThread constructors take 8~11 parameters,
which is too many. This patch moves private struct WorkerThreadStartupData
out of WorkerThread.cpp into its own public file, and uses it to pass around
Worker startup parameters.

BUG=257349

Review URL: https://chromiumcodereview.appspot.com/19384008

git-svn-id: svn://svn.chromium.org/blink/trunk@154640 bbb929c8-8fbe-4397-9dbb-9b2b20218538