Invoke LeakSanitizer in BrowserMainRunnerImpl::Shutdown().

We call LeakSanitizer in BrowserProcessImpl::ReleaseModule to avoid
shutdown-only leaks, but this doesn't work for processes that never create a
BrowserProcess object (e.g. in content_browsertests). Add another call to LSan
to cover this case.

BUG=307043
R=sky@chromium.org

Review URL: https://codereview.chromium.org/78053004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236310 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236310