Unbreak PrerenderLocationReplaceGWSHistograms.

Forgot to wait for the page to finish loading before running DidDisplayPass.
Instead, have NavigationOrSwapObserver wait for the target WebContents to
finish loading, for consistency with the non-swap case. This way we can avoid
the error-prone manual WaitForLoadStop calls.

BUG=341570

Review URL: https://codereview.chromium.org/133213018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249573 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249573