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In R39, ChromeOS rsyslogd no longer includes the hostname
field. Update SyslogParser accordingly.

While there:
- Update example shill log message to include process ID.
  (ChromeOS syslog recently started recording process IDs
  whenever possible.)
- Add a test log message from the kernel. This message does
  not include a process ID.
- Fix parsing of messages that do not have a process ID.
  Previously, for such messages, the process name would include
  a trailing ':'. With this change, they no longer do. (The
  process name will now be 'kernel', instead of 'kernel:'.)
- Simplify string EXPECTations. (No need for cstr() and STREQ.)

BUG=403558
TEST=unit test (ExtensionSyslogParserTest.ParseLog)

Review URL: https://codereview.chromium.org/486873005

Cr-Commit-Position: refs/heads/master@{#292058}