Clear return value of vapi.js content script (#3721)

Upon start-up, uBlock runs all content scripts in manifest.json using
`chrome.tabs.executeScript`. When this API is used, the value of the
last expression is automatically cloned and transferred to the
callback of `chrome.tabs.executeScript`. This is convenient if needed,
and a performance burden otherwise (the latter is the case for uBlock).

There are three content scripts that need to be checked:

- vapi.js
  The last expression is often the vAPI object, and it is relatively
  expensive to clone this object. This commit sets the value of the
  last expression to `void 0` to solve this inefficiency.

- vapi-client.js
  No action needed yet; The last expression is `vAPI.shutdown.add(...)`,
  which has a void return value.

- contentscript.js
  No action needed yet; The last expression is an immediately-invoked
  function expression without return value.

platform/chromium/vapi.js

@@ -53,4 +53,8 @@ if (
         : { uBO: true };
 }
 
+// Set the value of the last expression to undefined to avoid serializing the
+// vAPI object when the content script is run using chrome.tabs.executeScript.
+void 0;
+
 /******************************************************************************/