Add support for animations in vector icons.

ui/gfx/vector_icon_types.h

Index: ui/gfx/vector_icon_types.h
diff --git a/ui/gfx/vector_icon_types.h b/ui/gfx/vector_icon_types.h
index b18000b41fe304b8ade4ae9bdde408243741b068..26545275718a3eaad9bb08df5535e992ac84ef40 100644
--- a/ui/gfx/vector_icon_types.h
+++ b/ui/gfx/vector_icon_types.h
@@ -58,6 +58,10 @@ enum CommandType {
   // Flips the x-axis in RTL locales. Default is false, this command sets it to
   // true.
   FLIPS_IN_RTL,
+  // Defines a timed transition for other elements.

[tdanderson, 2017/05/08 21:22:54]

Consider providing a mini-explainer here, since unlike the commands on lines
35-50, these do not map 1:1 to something I can easily find in the SVG spec. I
would suggest highlighting: 

* the fact that the verbs and argument counts must be identical in the _FROM and
_TO blocks
* specify the arguments for _END
* clarify that the first two arguments for _END are relative to the same t=0
starting time, i.e., they are not relative to the ending time of the previous
transition.

[Evan Stade, 2017/05/08 22:27:42]

The doc I shared kinda explains this, and I plan to copy over parts of that into
the developer page at
https://www.chromium.org/developers/how-tos/vectorized-icons-in-native-chrome-ui
(which should itself maybe be ported to a README.md). But I didn't want to do
that before the code was landed and in use and we had some degree of certainty
it was not going to change further.

[tdanderson, 2017/05/09 20:31:40]

This sg, so long as the plan is to have the above points explained in the
developer page.

+  TRANSITION_FROM,
+  TRANSITION_TO,
+  TRANSITION_END,
   // Marks the end of the list of commands.
   END
 };