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When a navigation preload request is aborted, no console message is
sent to the service worker. That's because the "unsanitized_message"
field of blink::WebServiceWorkerError is not set.

When Sending the message to console, prefer the unsanitized one if
avaiable. Update service worker browser test to cover this change.

Bug: 877870
Test: content_browsertests  -gtest_filter=ServiceWorkerNavigationPreloadTest.CanceledByInterceptor
Change-Id: Icad28ce1a9feb40b729bc2504d9c9adb33593a0a
Reviewed-on: https://chromium-review.googlesource.com/1235376
Commit-Queue: Leon Han <leon.han@intel.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593823}