Add LayoutTest for MixedContentChecker::LogToConsoleAboutFetch() in dedicated workers

There are several LayoutTests under http/tests/security/mixedContent/ which
check the behavior of Mixed Content in pages.

I'm trying to implement WorkerFetchContext::ShouldBlockFetchByMixedContentCheck()
for off-main-thread-fetch without changing the existing behavior.

But unfortunately we don't have LayoutTests of Mixed Content for fetch API in
workers. So this CL adds LayoutTests for it before implementing it.

The -expected.txt files under virtual/off-main-thread-fetch/ in this CL don't
have CONSOLE logs of Mixed Content. But this is not the actual expected
behavior. When we will implement ShouldBlockFetchByMixedContentCheck() correctly
we should print the CONSOLE logs of Mixed Content same as the non-virtual tests.

And also, the result of "allowed" test under virtual/off-main-thread-fetch
should be "Result: LOADED".

I have another CL which adds browser_tests which involve the use action of
allowing insecure content that is not available in LayoutTests.
https://chromium-review.googlesource.com/c/576602/

Bug: 443374
Change-Id: I4dd1e6e12b51d478e68e6bb74c9beb9edb2c99e6
Reviewed-on: https://chromium-review.googlesource.com/577984
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489246}
9 files changed