Make leak counters thread-safe and turn them on by default for Debug

Make leak counters implemented with SK_DECLARE_INST_COUNT thread-safe.
Enable the leak counting for Debug builds when Skia is built as a
static library. Having SK_DECLARE_INST_COUNT without
SK_DEFINE_INST_COUNT relies on static variables in member functions
declared in the header files. These might be duplicated in the clients
of the library when Skia is built as a dynamic library, producing
incorrect operation.

Protect the instance counter initialization step (initStep) by
using SkOnce.

Makes SkOnce.h part of the public API, since SkInstCnt is public.

Protect the per-class child list shared variable with a per-class mutex.

Changes the behavior in the way that if the child list has been
"cleaned up", it will still try to create subsequent child lists.

BUG=skia:1219
R=robertphillips@google.com, mtklein@google.com, bsalomon@google.com, bungeman@google.com, djsollen@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/99483003

git-svn-id: http://skia.googlecode.com/svn/trunk@13120 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/convexpaths.cpp b/gm/convexpaths.cpp
index 8eb4cba..91afbfe 100644
--- a/gm/convexpaths.cpp
+++ b/gm/convexpaths.cpp
@@ -9,9 +9,9 @@
 #include "SkRandom.h"
 #include "SkTArray.h"
 
-class SkOnce : SkNoncopyable {
+class SkDoOnce : SkNoncopyable {
 public:
-    SkOnce() { fDidOnce = false; }
+    SkDoOnce() { fDidOnce = false; }
 
     bool needToDo() const { return !fDidOnce; }
     bool alreadyDone() const { return fDidOnce; }
@@ -27,7 +27,7 @@
 namespace skiagm {
 
 class ConvexPathsGM : public GM {
-    SkOnce fOnce;
+    SkDoOnce fOnce;
 public:
     ConvexPathsGM() {
         this->setBGColor(0xFF000000);