Web Inspector: Zoom never goes back to 100% size

Web Inspector: Zoom never goes back to 100% size
https://bugs.webkit.org/show_bug.cgi?id=150637

Add and subtract 20% from the zoom level so the zoom factor can return
to the 1 starting zoom factor if you go back the other direction.

Reviewed by Joseph Pecoraro.

* UserInterface/Base/Main.js:
(WebInspector._increaseZoom): Add 0.2 instead of multiplying 1.2.
(WebInspector._decreaseZoom): Subtract 0.2 instead of multiplying 0.8.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191697 268f45cc-cd09-0410-ab3c-d52691b4dbfc