GitLab

- Move the majority of set_process_reuse_policy() usage into creation
functions since most calls are made almost immediately after a Create
call.

- Created SiteInstanceImpl::CreateReusableInstanceForTesting() to
  explicitly mark instances where tests are creating SiteInstances
  that they expect to be able to reuse a process AND are expecting to
  not ever receive a default SiteInstance. This method will make it easier
  to avoid breaking tests when SiteInstanceImpll::CreateForURL() is
  updated to allow the default SiteInstance to be returned.

- Minor changes to make sure set_process_reuse_policy() is never called
  on the default SiteInstance.

Bug: 958060
Change-Id: I3e6a2dc9eb1f3f2abaccc690b2d8b1edd7e2b182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1603413Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Aaron Colwell <acolwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659307}