Set the BrowserAccessibilityManagerWin instance in the LegacyRenderWidgetHostHWND class in the ctor of the BrowserAccessibilityManagerWin class

The current code was passing the BrowserAccessibilityManagerWin instance in the MaybeCallNotifyWinEvent function which gets invoked if there is an Accessible client around. 

This leads to a crash during shutdown as the BrowserAccessibilityManagerWin class in its dtor dereferences a deleted LegacyRenderWidgetHostHWND class.
The LegacyRenderWidgetHostHWND instance is destroyed when the RenderWidgetHostViewAura instance is destroyed.

BUG=342160
TBR=cpu

Review URL: https://codereview.chromium.org/150713004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249951 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249951