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This introduces WorkerReportingProxy::willEvaluateWorkerScript() and removes
WorkerReportingProxy::didLoadWorkerScript().

Before this CL, didLoadWorkerScript() was called during WorkerThread
initialization. This timing was late, but it was intended because one of
observers (ServiceWorker) wanted to access WorkerGlobalScope in the notification
that is created during WorkerThread initialization in order to record size of
worker scripts.

After this CL, recording the size is done in willEvaluateWorkerScript() that
would be more appropriate timing for the operation. willEvaluateWorkerScript()
is also useful for unit tests. It enables them to wait until the timing that
they really wait for.

BUG=640843

Review-Url: https://codereview.chromium.org/2355723005
Cr-Commit-Position: refs/heads/master@{#420546}