utility/timer: Callback should pass in the timer

This makes the timer better resemble the other source types, and makes
it easier to access timer properties from the callback.

Since we do not yet have any committed users of this API it is safe to
change it.

Tested:
    Unit tests were run and builds work fine.

Change-Id: I9ecb24a20bc822b2ab55900e47a7cae834ba873d
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/utility/timer.cpp b/test/utility/timer.cpp
index c919f7e..414ad82 100644
--- a/test/utility/timer.cpp
+++ b/test/utility/timer.cpp
@@ -91,7 +91,7 @@
             .WillRepeatedly(DoAll(EventUnref(), Return(nullptr)));
         Event event(expected_event, &mock);
 
-        auto runCallback = [&]() {
+        auto runCallback = [&](TestTimer&) {
             if (callback)
             {
                 callback();