Mojo: Add GetMinimumLogLevel() to the logger interface.

Mojo: Add GetMinimumLogLevel() to the logger interface.

We'll want this to be able to implement efficient, base/logging.h-like
macros (with the similar behavior -- i.e., don't evaluate the stream if
it's not going to be logged).

R=darin@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277779

[viettrungluu, 2014-06-16 22:45:13]

(Dependent on https://codereview.chromium.org/331143007/, which remove
base::logging.*'s "error report" log severity.)

[darin (slow to review), 2014-06-16 23:58:22]

LGTM

[viettrungluu, 2014-06-17 16:43:23]

Committed patchset #2 manually as r277779.