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Currently the credentials mode of the FetchEvent's request is always 'omit'.
But this should be 'same-origin' or 'include'.
ex:
- <img src="./test.png">
 mode: no CORS
 credentials mode: same-origin
- <img src="./test.png" crossOrigin="anonymous">
 mode: CORS
 credentials mode: same-origin
- <img src="./test.png" crossOrigin="use-credentials">
 mode: CORS
 credentials mode: include

[1/2] blink: https://codereview.chromium.org/610403002/
[2/2] chromium: https://codereview.chromium.org/615493003/

This value is passed from the renederer to the ServiceWorker in the following steps.

In the renederer process:
  blink::ResourceRequest::setFetchCredentialsMode()  // not called yet
 blink::ResourceRequest::m_fetchCredentialsMode
  content::WebURLLoaderImpl::Context::Start()
   blink::WebURLRequest::fetchCredentialsMode()
    GetFetchCredentialsMode()
 conetnt::RequestInfo::fetch_credentials_mode

In the browser process:
 conetnt::RequestInfo::fetch_credentials_mode
  content::ResourceDispatcherHostImpl::BeginRequest()
   content::ServiceWorkerRequestHandler::InitializeHandler()
    content::ServiceWorkerProviderHost::CreateRequestHandler()
     content::ServiceWorkerControlleeRequestHandler::ServiceWorkerControlleeRequestHandler()
 content::ServiceWorkerControlleeRequestHandler::request_credentials_mode_
  content::ServiceWorkerControlleeRequestHandler::MaybeCreateJob()
   content::ServiceWorkerURLRequestJob::ServiceWorkerURLRequestJob()
 content::ServiceWorkerURLRequestJob::request_credentials_mode_
  content::ServiceWorkerURLRequestJob::CreateFetchRequest()
 content::ServiceWorkerFetchRequest::credentials_mode

In the ServiceWorker process:
 content::ServiceWorkerFetchRequest::credentials_mode
   content::ServiceWorkerScriptContext::OnFetchEvent()
   GetBlinkFetchRequestCredentialsMode()
   blink::WebServiceWorkerRequest::setCredentialsMode()
 blink::WebServiceWorkerRequest::WebServiceWorkerRequestPrivate::m_credentialsMode
  blink::Request::create()
  blink::FetchRequestData::create()
 blink::FetchRequestData::m_credentials

BUG=418509

Review URL: https://codereview.chromium.org/615493003

Cr-Commit-Position: refs/heads/master@{#297812}