Memory leak due to bad destroy sequence

During the destroy sequence, it happens that the mWebViewClassic was
set to null in the transferMessages function and consequently, the
call to WebCoreThreadWatchdog.unregisterWebView(mWebViewClassic) is
not really removing the view from the watchdog's list; creating a
memory leak.

picked from master branch https://android-review.googlesource.com/#/c/48087/

Change-Id: I2bae7c8d7c473e2af25e62a485699f88269d6658
Author: Jean-Christophe PINCE <jean-christophe.pince@intel.com>
Signed-off-by: Jean-Christophe PINCE <jean-christophe.pince@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 57869