[gwt-contrib] Adds {moz,webkit}RequestAnimationFrame support to animations. (issue1355805)

[t . broyer]

Reviewers: jlabanca,

Description:
Adds {moz,webkit}RequestAnimationFrame support to animations.
Refactor Animation with different implementations, adding
mozRequestAnimationFrame and webkitRequestAnimationFrame support in
addition to the timer-based implementation.

I verified non-regression by launching the test in HtmlUnit, and
manually checked in Firefox 4b11 and Chrome 11-dev (-runStyle Manual,
with plain old alert()s to check that the correct implementation is
being picked up).

ALso adds run() overloads taking an 'element' argument, that visually
"scopes" the animation (so that browsers can, as an optimization, skip
steps when the element is not visible to the user). I'll follow on with
a patch updating widgets so they pass the additional 'element' argument
when possible.

Please review this at http://gwt-code-reviews.appspot.com/1355805/show

Affected files:
  M user/src/com/google/gwt/animation/Animation.gwt.xml
  M user/src/com/google/gwt/animation/client/Animation.java
  A user/src/com/google/gwt/animation/client/AnimationImpl.java
  A user/src/com/google/gwt/animation/client/AnimationImplMozAnimTiming.java
  A user/src/com/google/gwt/animation/client/AnimationImplTimer.java
  A  
user/src/com/google/gwt/animation/client/AnimationImplWebkitAnimTiming.java


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