[MV2] Temporary fix for the unique tab feature

The unique tab feature doesn't work correctly in the MV2 variant of the
extension.

This CL fixes this by changing how the data managed by the
sessionStorage class is stored in the MV2 variant:

- Before, it was saved in the window object, which was erased each time
  the background script was turned inactive by the browser. This
  behavior was fine for most data, but not for the "translatorTab" data,
  which is why there was this bug.
- Now, it's saved in the persisted "local" storage area. For more
  information about this behavior, read the comment added to the
  //src/common/sessionStorage_mv2.js file.

Fixed: twpowertools:18
Change-Id: Ie3979937f4bd0699b57506666beace3ffe00172a
diff --git a/src/background.ts b/src/background.ts
index b0af7de..2db72b9 100644
--- a/src/background.ts
+++ b/src/background.ts
@@ -93,7 +93,7 @@
           chrome.windows.create({type: 'popup', url, width: 1000, height: 382});
         } else {
           chrome.tabs.create(newTabOptions, tab => {
-            ExtSessionStorage.set({translatorTab: tab.id});
+            ExtSessionStorage.set({translatorTab: tab?.id});
           });
         }
       })