Fix Flash loading in Private window

Also address #3835 by not
allowing Flash to be allowed persistently.

Auditors: @sririambv

Test Plan:
1. open http://homestarrunner.com/ in a private tab
2. right click to allow Flash once
3. flash should be allowed
4. close the private tab
5. open a regular tab and open http://homestarrunner.com/. flash should be blocked.

js/components/main.js

@@ -715,10 +715,6 @@ class Main extends ImmutableComponent {
   }
 
   get allSiteSettings () {
-    const activeFrame = FrameStateUtil.getActiveFrame(this.props.windowState)
-    if (activeFrame && activeFrame.get('isPrivate')) {
-      return this.props.appState.get('temporarySiteSettings')
-    }
     return this.props.appState.get('siteSettings')
   }
 

js/contextMenus.js

@@ -778,13 +778,16 @@ function mainTemplateInit (nodeProps, frame) {
       click: () => {
         appActions.changeSiteSetting(pageOrigin, 'flash', 1)
       }
-    }, {
-      label: locale.translation('allowFlashAlways'),
-      click: () => {
-        const expirationTime = Date.now() + 7 * 24 * 3600 * 1000
-        appActions.changeSiteSetting(pageOrigin, 'flash', expirationTime)
-      }
     })
+    if (!frame.get('isPrivate')) {
+      template.push({
+        label: locale.translation('allowFlashAlways'),
+        click: () => {
+          const expirationTime = Date.now() + 7 * 24 * 3600 * 1000
+          appActions.changeSiteSetting(pageOrigin, 'flash', expirationTime)
+        }
+      })
+    }
     return template
   }