Allow origin lock for WebUI pages.

Returning true for WebUI pages in DoesSiteRequireDedicatedProcess helps
to keep enforcing a SiteInstance swap during chrome://foo ->
chrome://bar navigation, even after relaxing
BrowsingInstance::GetSiteInstanceForURL to consider RPH::IsSuitableHost
(see https://crrev.com/c/783470 for that fixes process sharing in
isolated(b(c),d(c)) scenario).

I've manually tested this CL by visiting the following URLs:
- chrome://welcome/
- chrome://settings
- chrome://extensions
- chrome://history
- chrome://help and chrome://chrome (both redirect to chrome://settings/help)

Bug: 510588, 847127
Change-Id: I55073bce00f32cb8bc5c1c91034438ff9a3f8971
Reviewed-on: https://chromium-review.googlesource.com/1237392
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595259}
  void VerifyProcessIsBackgrounded(WebContents* web_contents) {
    constexpr bool kExpectedIsBackground = true;
    VerifyProcessPriority(web_contents->GetMainFrame()->GetProcess(),
                          kExpectedIsBackground);
  }

  void VerifyProcessIsBackgrounded(WebContents* web_contents) {
    constexpr bool kExpectedIsBackground = true;
    VerifyProcessPriority(web_contents->GetMainFrame()->GetProcess(),
                          kExpectedIsBackground);
  }
