D-Bus spec says bool values other than 1 and 0 are invalid.
Although C++'s int->bool conversion converts all values other than 0 to
1, it's still possible to create bool variables with values other than 1
and 0.
For example, when a variable is uninitialized, or with code like this:
  bool b;
  reinterpret_cast<int&>(b) = 10;

BUG=767024
TEST=dbus_unittests

Change-Id: I225456cb2662cffe992231f8f38a5add0dd2c250
Reviewed-on: https://chromium-review.googlesource.com/678537


Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503995}