change leftButton init position

Summary:
In navigationBar, when back to the previous view, the left button of the previous view should fade in like iOS native navBar, instead of move in. It's interesting that the right button acts correct.

React Native:

![clear](https://cloud.githubusercontent.com/assets/2622602/15535443/fa5e77bc-229f-11e6-8632-0dd9f5f2fb26.gif)

iOS Native:

![clear2](https://cloud.githubusercontent.com/assets/2622602/15535452/05999918-22a0-11e6-877c-63590b7bb9ef.gif)
Closes #7745

Differential Revision: D3392307

Pulled By: nicklockwood

fbshipit-source-id: e5384140049bf7b6f78fc093209ee48814a2ffdc

Libraries/CustomComponents/Navigator/NavigatorNavigationBarStylesIOS.js

@@ -77,7 +77,7 @@ var BASE_STYLES = {
 var Stages = {
   Left: {
     Title: merge(BASE_STYLES.Title, { left: -SCREEN_WIDTH / 2, opacity: 0 }),
-    LeftButton: merge(BASE_STYLES.LeftButton, { left: -SCREEN_WIDTH / 3, opacity: 0 }),
+    LeftButton: merge(BASE_STYLES.LeftButton, { left: 0, opacity: 0 }),
     RightButton: merge(BASE_STYLES.RightButton, { opacity: 0 }),
   },
   Center: {