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This CL:
* removes one not-well-documented method from WebServiceWorkerContextClient
* is a preparation for a potential fix of race condition in service worker
  extension

The context of the second bullet is a bit complicated. Service worker
background extension has following ordering assumption at initialization:
1. ExtensionMsg_Loaded is received on renderer
   extensions::Dispatcher::OnLoaded()
2. Installing extension API bindings
   extensions::Dispatcher::DidInitializeServiceWorkerContextOnWorkerThread()

However there is no such guarantees. The current implementation
happens to be working just because creating EmbeddedWorkerInstanceClient
is bound to RenderThreadImpl and StartWorker() mojo message is handled
after ExtensionMsg_Loaded.

To make sure the above ordering we need to wait for the loaded message
before installing extension API bindings. DidInitializeWorkerContext()
isn't a right place to install extension API bindings because it is called
via blink::WorkerThread::InitializeOnWorkerThread(), which doesn't wait
for anything after StartWorker() mojo message is received.

Fortunately we have blink::WorkerGlobalScope::ReadyToRunScript(). This
was introduced for service worker update to suspend service worker
execution after script fetch. We could probably use this to wait for
the extension loaded IPC message. ReadyToRunScript() calls
WillEvaluateScript() before executing the script so this would be a
better place to install extension API bindings.

Bug: 995134
Change-Id: I54d44391149225c5c52294bcca32a86705a9c0a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775856
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692997}