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/gm/video_decoder.cpp b/gm/video_decoder.cpp
index 876dd48..e6785b8 100644
--- a/gm/video_decoder.cpp
+++ b/gm/video_decoder.cpp
@@ -55,7 +55,7 @@
         }
     }
 
-    bool onAnimate(const AnimTimer& timer) override {
+    bool onAnimate(double nanos) override {
         return true;
     }