[Windows] Migrate error logging to FML_LOG

[cbracken]

Migrates error logging from logging directly to stderr to using the
FML_LOG macro with a specified log level.

No additional tests since there is no semantic change to the logging
(FML_LOG simply writes to stderr on desktop).

Pre-launch Checklist

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

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[yaakovschectman]

Does FML_LOG not require a newline? I notice the old std::cerr usages get an std::endl at the end, but there's nothing corresponding in the changed versions.

[loic-sharma]

@yaakovschectman It adds a new line to the stream automatically:

engine/fml/logging.cc

Lines 75 to 77 in 643742e

	#if !defined(OS_FUCHSIA)
	stream_ << std::endl;
	#endif

[loic-sharma]

LGTM with minor nits

[Hixie]

test-exempt: code refactor with no semantic change