Subzero: Use C++11 member initializers where practical.

Also change the pattern "foo() {}" into "foo() = default;" for ctors and dtors.

Generally avoids initializing unique_ptr<> members to nullptr in a .h file, because that requires knowing the definition of the underlying class which may not be available to all includers.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1197223002
diff --git a/src/IceTimerTree.cpp b/src/IceTimerTree.cpp
index f1366d5..6c92215 100644
--- a/src/IceTimerTree.cpp
+++ b/src/IceTimerTree.cpp
@@ -20,8 +20,7 @@
 namespace Ice {
 
 TimerStack::TimerStack(const IceString &Name)
-    : Name(Name), FirstTimestamp(timestamp()), LastTimestamp(FirstTimestamp),
-      StateChangeCount(0), StackTop(0) {
+    : Name(Name), FirstTimestamp(timestamp()), LastTimestamp(FirstTimestamp) {
   if (!ALLOW_DUMP)
     return;
   Nodes.resize(1); // Reserve Nodes[0] for the root node (sentinel).