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* PrerenderRenderFrameObserver::SetIsPrerendering() was only calling
  PrerenderHelper::SetIsPrerendering() when mode != kNoPrerender,
  and the function just early returns in that case. Therefore, delete
  PrerenderHelper::SetIsPrerendering().
* Also mark some code only used by tests as test-only code.

This is a bit uncomfortable since we might be needing this code
as prerender gets revisited. However, I found it confusing that the
caller was calling the callee only when the callee ignores the call,
so think it's constructive to simplify this.

This also reveals that PrerenderURLLoaderThrottle::PrerenderUsed() is
never called, but I won't traverse into there since we might want the
code.

Bug: 755921

Change-Id: I486e78066350c8a20c63fdca3feb1344766c6b15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2396057
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805225}