feat: file logging and configurable minimum log level

[jpnurmi]

Required for (and built by):

• feat: add --log-file and --log-level options crashpad#148
• feat(crashpad): capture client & handler logs sentry-native#1658

Two additions to mini_chromium's logging:

1. LOG_TO_FILE destination. Implements the previously stubbed LOG_TO_FILE destination and adds a simplified base::OpenFile() helper (after upstream Chromium's helper of the same name). Callers pass the path via LoggingSettings::log_file_path. The file is opened lazily for appending on the first emitted message and held by a std::unique_ptr<FILE> with a silent closer, so an fclose failure during static destruction cannot re-enter Flush() and deadlock on the file lock. File state (lock, path, handle, and an enabled flag) is grouped in a file-scope LogFile struct; an empty path or a failed open is handled by clearing enabled, so a transient open failure cannot clobber a concurrent InitLogging(). Concurrent LOG calls are serialized with a base::Lock.

2. Configurable minimum log level. Replaces the hardcoded GetMinLogLevel() with a runtime-configurable value. Adds SetMinLogLevel() and a min_log_level field on LoggingSettings (applied by InitLogging). Default remains LOG_INFO, so existing callers are unaffected.

g_logging_destination and g_min_log_level are std::atomic so reads on the LOG hot path don't need the file lock and writes from InitLogging / SetMinLogLevel don't race concurrent readers.

See also:

• Crashpad: improve logging for debugging purposes sentry-native#1468

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