tools: separate TimeUtils from AnimTimer

gm, slides, and samples no longer need to know about the implementation
details of AnimTimer.

This
    virtual bool onAnimate(const AnimTimer&);
becomes this:
    virtual bool onAnimate(double /*nanoseconds*/);
which is much easier to reason about.

AnimTimer itself is now part of viewer.

Change-Id: Ib70bf7a0798b1991f25204ae84f70463cdbeb358
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/226838
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 8e2ec15..6fba35f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1650,7 +1650,7 @@
       "tools/fonts/TestTypeface.h",
       "tools/fonts/ToolUtilsFont.cpp",
       "tools/random_parse_path.cpp",
-      "tools/timer/AnimTimer.h",
+      "tools/timer/TimeUtils.h",
       "tools/timer/Timer.cpp",
     ]
     libs = []
@@ -2423,6 +2423,7 @@
   test_app("viewer") {
     is_shared_library = is_android
     sources = [
+      "tools/viewer/AnimTimer.h",
       "tools/viewer/BisectSlide.cpp",
       "tools/viewer/GMSlide.cpp",
       "tools/viewer/ImGuiLayer.cpp",