Ensure there is only one prerender PendingSwap per target WebContents.

Add a test for this. Also add another test to further improve PendingSwap's
test coverage. Restore the behavior where PendingSwaps get removed when
they move to to_delete_prerenders_ and revise the long comment about the
this-deleting consequences of this.

BUG=338374
TEST=PrerenderBrowserTest.PrerenderPendingSwapNewNavigation,
     PrerenderBrowserTest.PrerenderDoublePendingSwap

Review URL: https://codereview.chromium.org/146833013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248036 0039d316-1c4b-4281-b951-d872f2087c98
Replace hide key image with "hide"

BUG=339216

Review URL: https://codereview.chromium.org/135183017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248037 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248037