Increase z-index

Summary:
Ensure that sheets aren't underneath the headers in the images plugin.

{F162094893}

Reviewed By: jknoxville

Differential Revision: D15853171

fbshipit-source-id: 8fd7217a50ee2823a89b788718fdef50830dc31a

src/__tests__/__snapshots__/App.electron.js.snap

@@ -113,7 +113,7 @@ exports[`Empty app state matches snapshot 1`] = `
     </div>
   </div>
   <div
-    className="css-6xnt2s"
+    className="css-v6xy5u"
   >
     <div
       id="pluginSheetContents"

src/chrome/Sheet.js

@@ -26,7 +26,7 @@ const DialogContainer = styled('div')(({state}) => ({
   position: 'absolute',
   left: '50%',
   top: 38,
-  zIndex: 3,
+  zIndex: 5,
   backgroundColor: '#EFEEEF',
   border: '1px solid #C6C6C6',
   borderTop: 'none',