Have the unload event execute in background on cross-site navigations

Cross-site navigations trigger renderer swap. This CL makes it so that the swap
does not need for the old renderer unload event to be fired. Instead, it will
be executed in the background, and the new renderer will be swapped in
immediately.
BUG=323528

Review URL: https://codereview.chromium.org/88503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249676 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249676