libchromeos: void OsReleaseReader.Load()

Previously OsReleaseReader returned a bool, which meant that code that
called load wanted to CHECK the return values and possibly log warning
messages, etc. This was unecessary since OsReleaseReader.Load never
returns false.

TEST=unittests
BUG=none

Change-Id: I7064c6a788897b5d5c687d6c9c5f4e03d4ca21a7
Reviewed-on: https://chromium-review.googlesource.com/223990


Reviewed-by: Bertrand Simonnet <bsimonnet@google.com>
Commit-Queue: Nathan Bullock <nathanbullock@google.com>
Tested-by: Nathan Bullock <nathanbullock@google.com>