Revert 256396 "Add views implementation for AXTreeSource."

This is a speculative revert of Linux ASAN/LSAN tests.

> Add views implementation for AXTreeSource.
> 
> Note that this cl required fixing PanelView (chrome/browser/ui/views/panels). It neglected to remove its observers previously leading to av's when attempting to notify observers from |Widget|.
> 
> 
> BUG=318758
> 
> Review URL: https://codereview.chromium.org/173893003

TBR=dtseng@chromium.org

Review URL: https://codereview.chromium.org/196653002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256411 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256411