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Process-per-site makes all WebUI NTPs share a single renderer process.
This is what the local NTP does and has the following performance
advantages (measured on my linux machine):

* Reduces mean of NewTabPage.LoadTime.WebUINTP (duration from starting
  the NTP navigation until the most visited items are added to the DOM)
  from 271ms to 141ms.

* Reduces memory overhead of opening a new NTP from ~30MB to ~5MB.

NOTE: The aforementioned performance improvements only hold true if
another NTP is open to have the process pre-warmed. The WebUI NTP does
not seem to use the pre-warmed spare renderer.

+ Move existing WebUI NTP browser test to //c/b/ui/webui/new_tab_page.

Bug: 1075641
Change-Id: Id94294eac41e3ab083e8eece567941ef4d40d08b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220634
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773865}