[ReactNative] Don't activate Chrome when debugger is already open

Summary:
Before this diff every time you reload in debug mode Chrome window
is actiavated. Looks like that behaviour is pretty annoying.

Fixes #689

@public

Test Plan:
```
$ ./packager/launchChromeDevTools.applescript 'https://www.facebook.com/'
```

First time it opens a new tab and activates Chrome, running this again does
not activate Chrome if the tab already exists.

packager/launchChromeDevTools.applescript

@@ -11,7 +11,6 @@ on run argv
   set theURL to item 1 of argv
 
   tell application "Chrome"
-    activate
 
     if (count every window) = 0 then
       make new window
@@ -40,6 +39,7 @@ on run argv
       set theWindow's active tab index to theTabIndex
     else
       tell window 1
+        activate
         make new tab with properties {URL:theURL}
       end tell
     end if