Always use the observer's index in ObserverList.removeObserver().

BUG=349925

Review URL: https://codereview.chromium.org/179793007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255924 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255924