Temporarily disable benign ASSERT in Document::didRemoveTouchEventHandler

This ASSERT fires whenever an iframe containing a plugin has it's source changed,
but the result is harmless (just indicates incomplete tracking of what has already
been cleaned up elsewhere).  I will fix this properly but need to track down a
perf regression.  Disabling the assert temporarily since this is blocking
people working with PNaCL.

BUG=254203

Review URL: https://chromiumcodereview.appspot.com/22967010

git-svn-id: svn://svn.chromium.org/blink/trunk@156455 bbb929c8-8fbe-4397-9dbb-9b2b20218538