Improve performance and reliability of mutationObservers

Some mutationObservers were attaching to the |.scrollable-content|
element which might not exist when the script runs, so now they attach
to the body element instead.

Also, now the //content_scripts/console_inject.js's mutationObserver no
longer requests attribute changes, because they weren't being used,
which introduced a slight performance penalty.

Bug: #21

Change-Id: I7abca0bcd4eb80a98e3184933977d8116a9db0ec
diff --git a/src/injections/profileindicator_inject.js b/src/injections/profileindicator_inject.js
index ddf1cfe..1a0125f 100644
--- a/src/injections/profileindicator_inject.js
+++ b/src/injections/profileindicator_inject.js
@@ -352,8 +352,7 @@
   }
 
   mutationObserver = new MutationObserver(mutationCallback);
-  mutationObserver.observe(
-      document.querySelector('.scrollable-content'), observerOptions);
+  mutationObserver.observe(document.body, observerOptions);
 } else {
   // We are in TW
   authuser = (new URL(location.href)).searchParams.get('authuser') || '0';