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This CL avoids starting navigation for prerendering when it already
started for the same URL.

To do that, this moves PrerenderHost::StartNavigation() call into
PrerenderHostRegistry::RegisterHost(). This doesn't change behavior
around networking. Even before this change, the duplicate navigation
didn't issue a network request because PrerenderHost was destroyed right
after PrerenderHost::StartNavigation() was called. The browser tests
make sure this behavior.

In addition to that, this change renames RegisterHost() to
CreateAndStartHost() and then moves the PrerenderHost creation into the
function for code cleanup.

Bug: 1132746
Change-Id: Ibc1d51921d845c2746a1a0caca27e08e77256699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567514Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833583}