Don't enable "replace current entry" if there is no entry to replace.

BUG=457149
TEST=included; as in bug

Review URL: https://codereview.chromium.org/914893002

Cr-Commit-Position: refs/heads/master@{#316031}
(cherry picked from commit b6cc1564475f04f6598ac6a228c33ff3530d4450)

(This cherry-pick omits the test as it does not merge cleanly.)

Review URL: https://codereview.chromium.org/920343004

Cr-Commit-Position: refs/branch-heads/2272@{#325}
Cr-Branched-From: 827a380cfdb31aa54c8d56e63ce2c3fd8c3ba4d4-refs/heads/master@{#310958}

• content/browser/frame_host/navigation_controller_impl.cc[diff]
• content/browser/frame_host/navigation_controller_impl_unittest.cc[diff]
• content/renderer/render_frame_impl.cc[diff]
• content/test/data/navigation_controller/simple_page_1.html[Added - diff]