S13nSW: Set Content-Type for synthesized responses in content_browsertests

Some of content_browsertests are timing out because we haven't supported
mime-sniffing in S13nSW code path yet (issue 771118). These tests wait
for <title> being updated but it doesn't happen when a synthesized
response doesn't have 'Content-Type: text/html'. Probably a right fix
would be to support mime-sniffing but it's better to make these tests
independent from mime-sniffing as these tests do nothing with
mime-sniffing. This CL changes results of following tests:

- ServiceWorkerBrowserTest.FetchPageWithSaveData -> PASS
- ServiceWorkerBrowserTest.ResponseFromHTTPSServiceWorkerIsMarkedAsSecure -> CRASH
- ServiceWorkerBrowserTest.ResponseFromHTTPServiceWorkerIsNotMarkedAsSecure -> PASS

Following tests are still timing out because of unexpected behavior:

- ServiceWorkerBrowserTest.ImportsBustMemcache
- ServiceWorkerBrowserTest.Reload

Bug: 715640, 771118
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I14819f2b39bfa83edf9eb5fcbf9950d72fa5b292
Reviewed-on: https://chromium-review.googlesource.com/1067220
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560448}
4 files changed