Bug 1264830 - Part 4: Change nsStyleDisplay::mWillChange to use nsTArray. r=bholley

The initial value of nsStyleDisplay::mWillChange is represented by an
empty array, and will-change is not so common, so we change it to use an
nsTArray.

layout/style/nsStyleStruct.h

@@ -2550,7 +2550,7 @@ struct nsStyleDisplay
                                 // match mWillChange. Also tracks if any of the
                                 // properties in the will-change list require
                                 // a stacking context.
-  AutoTArray<nsString, 1> mWillChange;
+  nsTArray<nsString> mWillChange;
 
   uint8_t mTouchAction;         // [reset] see nsStyleConsts.h
   uint8_t mScrollBehavior;      // [reset] see nsStyleConsts.h NS_STYLE_SCROLL_BEHAVIOR_*